Orthopedic MCQs: Skeletal Dysplasias & Bone Development

Question 1

A 12-year-old girl presents with precocious puberty, irregular 'coast of Maine' café-au-lait spots, and a progressive proximal femur deformity. Radiographs reveal expansile, ground-glass lesions in the femur and pelvis. What is the underlying molecular pathogenesis of her skeletal condition?

Explanation

Correct Answer: Activating mutation of the GNAS1 gene leading to increased intracellular cAMP

This patient presents with McCune-Albright syndrome, characterized by the triad of polyostotic fibrous dysplasia, precocious puberty (or other endocrinopathies), and café-au-lait spots with irregular borders ('coast of Maine'). The underlying cause is a post-zygotic somatic activating mutation in the GNAS1 gene, which encodes the alpha subunit of the stimulatory G protein (Gs). This leads to constitutive activation of adenylate cyclase and elevated intracellular cyclic AMP (cAMP), resulting in abnormal proliferation and differentiation of preosteoblasts.

Question 2

A 25-year-old male presents with multiple asymmetric cartilaginous lesions in the tubular bones of his hands and feet, accompanied by multiple bluish, compressible subcutaneous nodules. Which of the following is the most significant long-term complication associated with this specific syndrome?

Explanation

Correct Answer: High risk of malignant transformation to chondrosarcoma and visceral malignancies

The clinical presentation of multiple enchondromas associated with soft tissue hemangiomas (bluish subcutaneous nodules) is diagnostic of Maffucci syndrome. Unlike Ollier disease (multiple enchondromatosis alone), Maffucci syndrome carries a significantly higher risk of malignant transformation. Patients have an estimated 20-30% (up to 100% in some long-term follow-up studies) risk of developing chondrosarcoma, as well as a high risk of developing visceral malignancies (e.g., astrocytomas, gastrointestinal malignancies).

Question 3

A 14-year-old boy with a known diagnosis of Multiple Hereditary Exostoses (MHE) presents with a progressive deformity of his left forearm. Which of the following is the most typical pattern of forearm deformity expected in this condition?

Explanation

Correct Answer: Ulnar shortening, radial bowing, and potential radial head dislocation

In Multiple Hereditary Exostoses (Osteochondromatosis), forearm deformities are common due to the presence of osteochondromas affecting the distal physes. The distal ulnar physis contributes a larger percentage to the overall length of the ulna compared to the radius. Therefore, an osteochondroma here disproportionately retards ulnar growth, leading to relative ulnar shortening. The continued growth of the tethered radius results in radial bowing, increased ulnar variance (negative), and eventually radial head subluxation or dislocation (Masada classification).

Question 4

A 35-year-old female presents with chronic, severe pain and stiffness in her right lower extremity. Radiographs show dense, irregular, eccentric hyperostosis along the cortex of the femur and tibia, resembling 'dripping candle wax'. What is the most likely diagnosis?

Explanation

Correct Answer: Melorheostosis

Melorheostosis (Leri disease) is a rare, non-hereditary sclerosing bone dysplasia characterized by linear, eccentric cortical hyperostosis that classically resembles 'dripping candle wax' on radiographs. Unlike many other sclerosing dysplasias, melorheostosis is frequently symptomatic, presenting with severe pain, joint contractures, and soft tissue fibrosis. It is associated with somatic mutations in the MAP2K1 gene in many cases, though LEMD3 mutations have also been implicated in familial cases overlapping with osteopoikilosis.

Question 5

An 18-year-old patient with polyostotic fibrous dysplasia presents with a progressive 'shepherd's crook' deformity of the proximal femur and a recent pathological fracture. Which of the following is the most appropriate surgical stabilization principle for this patient?

Explanation

Correct Answer: Valgus osteotomy and stabilization with a load-sharing intramedullary device

The surgical management of fibrous dysplasia, particularly the 'shepherd's crook' deformity of the proximal femur, requires careful consideration of the abnormal bone biology. Cortical and cancellous bone grafts are typically resorbed and replaced by dysplastic bone, making simple curettage and grafting ineffective. Plate fixation is prone to failure due to the poor quality of the dysplastic bone and the massive bending forces. The gold standard is correction of the mechanical axis (valgus osteotomy) combined with a load-sharing intramedullary device (e.g., reconstruction nail) to protect the entire length of the affected bone.

Question 6

An incidental pelvic radiograph of a 40-year-old male reveals numerous small, well-defined, symmetric sclerotic foci clustered around the periarticular regions of the pelvis and proximal femurs. He is completely asymptomatic. If this patient has an associated syndromic skin manifestation, what is the most likely clinical finding?

Explanation

Correct Answer: Dermatofibromas (connective tissue nevi)

The radiographic description of multiple small, symmetric, periarticular sclerotic foci in an asymptomatic patient is classic for Osteopoikilosis ('spotted bone disease'). When osteopoikilosis is associated with skin lesions, specifically connective tissue nevi (dermatofibromas lenticularis disseminata), the condition is known as Buschke-Ollendorff syndrome. This syndrome is inherited in an autosomal dominant pattern and is associated with mutations in the LEMD3 gene.

Question 7

A 7-year-old child presents with a leg length discrepancy and angular deformity of the right lower extremity. Radiographs show multiple radiolucent expansile lesions in the metaphyses and diaphyses of the right femur and tibia, with linear striations. The left leg is unaffected. Which of the following statements regarding this condition is true?

Explanation

Correct Answer: It is associated with somatic mosaic mutations in the IDH1 or IDH2 genes

The clinical picture describes Ollier disease (multiple enchondromatosis), characterized by multiple enchondromas that are often unilateral or asymmetric, causing growth disturbances and deformities. Ollier disease is not hereditary; it is caused by somatic mosaic mutations in the isocitrate dehydrogenase 1 (IDH1) or IDH2 genes. Enchondromas typically do not cross the open physis. The risk of malignant transformation (usually to chondrosarcoma, not osteosarcoma) is approximately 5-30%, not >50%.

Question 8

A 45-year-old female with known polyostotic fibrous dysplasia presents with a painless, slow-growing soft tissue mass in her right thigh. MRI reveals a well-circumscribed intramuscular mass with high T2 signal and low T1 signal. What is the most likely diagnosis of the soft tissue mass?

Explanation

Correct Answer: Intramuscular myxoma

The association of polyostotic fibrous dysplasia with intramuscular myxomas is known as Mazabraud syndrome. These myxomas are benign, slow-growing, painless soft tissue tumors that typically present in middle age, often years after the diagnosis of fibrous dysplasia. On MRI, myxomas characteristically show low signal intensity on T1-weighted images and high signal intensity on T2-weighted images due to their high mucin and water content.

Question 9

A radiograph of a 20-year-old female taken for a minor knee sprain incidentally reveals dense, longitudinal striations in the metaphyses of the distal femur and proximal tibia. The patient is otherwise healthy and asymptomatic. This radiographic finding is characteristic of which of the following skeletal dysplasias?

Explanation

Correct Answer: Osteopathia striata

Osteopathia striata (Voorhoeve disease) is a benign, usually asymptomatic skeletal dysplasia characterized radiographically by dense, longitudinal linear striations in the metaphyses and diaphyses of long bones, and sometimes a fan-like appearance in the ilium. It is caused by mutations in the WTX (AMER1) gene and is inherited in an X-linked dominant pattern. It is often an incidental finding and requires no treatment.

Question 10

In the context of Multiple Hereditary Exostoses (MHE), which of the following cellular processes is primarily disrupted by the underlying genetic mutation?

Explanation

Correct Answer: Synthesis of heparan sulfate proteoglycans in the Golgi apparatus

Multiple Hereditary Exostoses (MHE) is caused by autosomal dominant mutations in the EXT1 or EXT2 genes. These genes encode glycosyltransferases localized to the Golgi apparatus, which are essential for the polymerization and synthesis of heparan sulfate proteoglycans (HSPGs). Deficiency in HSPGs disrupts normal Indian hedgehog (Ihh) signaling and chondrocyte proliferation/differentiation at the growth plate, leading to the formation of osteochondromas.

Question 11

A 12-year-old girl presents with a progressive 'shepherd's crook' deformity of the proximal femur and large, irregularly bordered hyperpigmented skin macules. Which of the following best describes the underlying molecular pathogenesis of her skeletal condition?

Explanation

Correct Answer: Gain-of-function mutation in the GNAS1 gene leading to increased intracellular cAMP

This patient presents with classic signs of McCune-Albright syndrome (polyostotic fibrous dysplasia, 'coast of Maine' cafe-au-lait spots, and endocrine abnormalities). Fibrous dysplasia is caused by a somatic, gain-of-function mutation in the GNAS1 gene. This mutation affects the alpha subunit of the stimulatory G protein (Gs-alpha), leading to constitutive activation of adenylate cyclase and elevated intracellular cyclic AMP (cAMP). This anarchic development results in the replacement of normal bone with fibrous stroma and immature woven bone. EXT1 mutations cause Multiple Hereditary Exostoses. FGFR3 mutations cause Achondroplasia. CBFA1 mutations cause Cleidocranial Dysplasia. IDH1/2 mutations cause Enchondromatosis (Ollier/Maffucci).

Question 12

A 25-year-old male with a history of multiple asymmetrical cartilaginous lesions in the phalanges and bluish subcutaneous nodules presents with a rapidly enlarging, painful mass in his right proximal humerus. Biopsy confirms chondrosarcoma. Which of the following is the most likely underlying syndrome, and what is the approximate lifetime risk of malignant transformation of his skeletal lesions?

Explanation

Correct Answer: Maffucci syndrome; up to 50% risk of chondrosarcoma

The presence of multiple enchondromas combined with soft tissue hemangiomas (bluish subcutaneous nodules) is pathognomonic for Maffucci syndrome. Both Ollier disease and Maffucci syndrome represent anarchic development of cartilage (enchondromatosis). However, Maffucci syndrome carries a significantly higher risk of malignant transformation compared to Ollier disease. The risk of chondrosarcoma in Maffucci syndrome is frequently cited as being between 30% and 50%, and the overall lifetime risk of any malignancy (including astrocytomas and GI malignancies) approaches 100%. Ollier disease has a lower malignant transformation risk (typically 5-30%). Multiple Hereditary Exostoses carries a 1-5% risk.

Question 13

A 10-year-old boy with multiple osteochondromas presents with a progressive forearm deformity. Which of the following is the classic pattern of forearm deformity expected in this patient due to the anarchic development of bone constituents?

Explanation

Correct Answer: Ulnar shortening, radial bowing, ulnar deviation of the hand, and potential radial head dislocation

Multiple Hereditary Exostoses (MHE), also known as diaphyseal aclasis, is characterized by the anarchic development of cartilage-capped bony outgrowths. In the forearm, the distal ulna is disproportionately affected because it contributes a large percentage of the bone's overall longitudinal growth and has a small cross-sectional area. This leads to relative ulnar shortening. The radius continues to grow, resulting in radial bowing to accommodate the length discrepancy. The tethering effect leads to ulnar deviation of the carpus and can eventually cause dislocation of the radial head (usually posterolaterally).

Question 14

A biopsy is taken from a lytic, expansile lesion in the proximal femur of a 15-year-old patient. The lesion has a 'ground-glass' appearance on radiographs. Which of the following histological descriptions is the hallmark of this condition?

Explanation

Correct Answer: Irregular trabeculae of woven bone lacking prominent osteoblastic rimming, set in a bland fibrous stroma

The clinical and radiographic description (lytic, expansile, 'ground-glass' appearance) is classic for Fibrous Dysplasia. Histologically, fibrous dysplasia is characterized by irregular, C- or W-shaped trabeculae of immature woven bone (often described as 'Chinese characters') that arise directly from a moderately cellular, bland fibrous stroma. A key distinguishing feature is the absence of prominent osteoblastic rimming around these bony trabeculae, reflecting the anarchic, arrested development of osteoblasts. Option C describes Giant Cell Tumor. Option D describes Ewing Sarcoma. Option E describes Paget disease of bone.

Question 15

A 7-year-old girl is diagnosed with Ollier disease after presenting with multiple enchondromas predominantly affecting the right side of her body. Which of the following statements regarding the genetics and transmission of her condition is most accurate?

Explanation

Correct Answer: It results from somatic mosaic mutations in the IDH1 or IDH2 genes and is not inherited

Ollier disease (enchondromatosis) is a non-hereditary disorder characterized by the anarchic development of multiple enchondromas, often with a unilateral predominance. It is caused by somatic mosaic mutations in the isocitrate dehydrogenase 1 (IDH1) or IDH2 genes. Because it is a somatic mosaicism occurring post-zygotically, it is not passed from parent to child. GNAS somatic mutations cause Fibrous Dysplasia. EXT1/EXT2 germline mutations cause Multiple Hereditary Exostoses (autosomal dominant).

Question 16

A 45-year-old female with known polyostotic fibrous dysplasia presents with a painless, slow-growing soft tissue mass in her right thigh. MRI reveals a well-circumscribed intramuscular lesion with high T2 signal intensity. Biopsy confirms an intramuscular myxoma. This combination of findings is pathognomonic for which of the following syndromes?

Explanation

Correct Answer: Mazabraud syndrome

Mazabraud syndrome is a rare disorder characterized by the association of fibrous dysplasia (usually polyostotic) and one or multiple intramuscular myxomas. The myxomas typically occur in the same anatomic region as the most severely affected bones. McCune-Albright syndrome involves fibrous dysplasia, cafe-au-lait spots, and endocrinopathies. Maffucci syndrome involves enchondromas and hemangiomas. Jaffe-Campanacci syndrome involves multiple non-ossifying fibromas and cafe-au-lait spots.

Question 17

A 22-year-old male with Multiple Hereditary Exostoses (MHE) is being evaluated in the clinic. He has a prominent osteochondroma on his distal medial femur. Which of the following findings is the strongest indication for prophylactic wide surgical excision of the lesion?

Explanation

Correct Answer: MRI demonstrates a cartilage cap thickness of 2.5 cm

In adults with osteochondromas, a cartilage cap thickness greater than 1.5 to 2.0 cm on MRI is highly suspicious for malignant transformation to secondary chondrosarcoma. This warrants wide surgical excision. While sessile lesions have a slightly higher risk of malignancy than pedunculated ones, cap thickness is the most reliable imaging indicator. Pain after extreme exertion is likely mechanical; however, new onset of pain at rest or growth of the lesion after skeletal maturity are clinical red flags. Osteochondromas naturally point away from the joint as they grow, which is a normal finding.

Question 18

A 30-year-old patient with polyostotic fibrous dysplasia complains of severe, persistent bone pain in the lower extremities that limits daily activities. Surgical intervention is not currently indicated for structural reasons. Which of the following pharmacological therapies has the best evidence for reducing bone pain in this condition?

Explanation

Correct Answer: Intravenous bisphosphonates (e.g., pamidronate or zoledronic acid)

In fibrous dysplasia, the anarchic fibrous tissue produces excess factors (like IL-6) that stimulate osteoclast activity, leading to bone resorption and pain. Intravenous bisphosphonates (such as pamidronate or zoledronic acid) are the mainstay of medical treatment for symptomatic fibrous dysplasia. They inhibit osteoclastic bone resorption, significantly reducing bone pain and potentially improving the radiographic appearance of the lesions. Teriparatide (an anabolic agent) is contraindicated in patients with an increased risk of osteosarcoma or active bone disorders like fibrous dysplasia.

Question 19

A 4-year-old boy presents with an asymmetric, painless swelling on the medial aspect of his right ankle. Radiographs show an irregular, ossified mass arising from the medial epiphysis of the distal tibia. The condition is characterized by an anarchic overgrowth of epiphyseal cartilage. What is the most likely diagnosis?

Explanation

Correct Answer: Dysplasia Epiphysealis Hemimelica (Trevor disease)

Dysplasia Epiphysealis Hemimelica, also known as Trevor disease, is a rare developmental disorder characterized by an asymmetric, anarchic overgrowth of cartilage at the epiphysis (essentially an intra-articular osteochondroma). It most commonly affects the medial side of the epiphyses in the lower extremities (ankle or knee) of young children. Multiple Epiphyseal Dysplasia presents with symmetric, delayed, and irregular ossification of multiple epiphyses. Ollier disease involves the metaphyses and diaphyses (enchondromas), not primarily an epiphyseal overgrowth.

Question 20

A 6-year-old girl is brought to the orthopedic clinic with a limp and a radiographically confirmed fibrous dysplasia lesion in her proximal femur. Physical examination reveals a large cafe-au-lait macule with irregular 'coast of Maine' borders on her trunk. If this patient has McCune-Albright syndrome, which of the following is the most common endocrine abnormality she is likely to experience?

Explanation

Correct Answer: Precocious puberty

McCune-Albright syndrome is defined by the triad of polyostotic fibrous dysplasia, cafe-au-lait macules, and autonomous endocrine hyperfunction. The most common endocrine manifestation, particularly in females, is gonadotropin-independent precocious puberty, caused by autonomous ovarian cyst formation and estrogen production. While hyperthyroidism, Cushing syndrome, and growth hormone excess (leading to acromegaly/gigantism) can also occur due to the underlying GNAS mutation affecting various endocrine glands, precocious puberty is by far the most frequent.

Question 21

What is the underlying molecular pathogenesis of fibrous dysplasia?

Explanation

Correct Answer: Mutation in GNAS leading to constitutive activation of Gs-alpha and increased intracellular cAMP.

Fibrous dysplasia is caused by a somatic, activating mutation in the GNAS gene. This gene encodes the alpha subunit of the stimulatory G protein (Gs-alpha). The mutation leads to constitutive activation of adenylate cyclase, resulting in high intracellular levels of cyclic AMP (cAMP). This overproduction of cAMP affects the differentiation of osteoblasts, leading to the replacement of normal bone and marrow with fibrous tissue and woven bone. FGFR3 mutations are seen in achondroplasia, EXT1 in multiple hereditary exostoses, and COMP in pseudoachondroplasia.

Question 22

A 25-year-old patient with multiple enchondromas and soft tissue hemangiomas presents for routine follow-up. Aside from chondrosarcoma, which of the following malignancies is this patient at the highest risk of developing compared to the general population?

Explanation

Correct Answer: Astrocytoma

The patient has Maffucci syndrome, characterized by multiple enchondromas and soft tissue hemangiomas. Patients with Maffucci syndrome have a significantly higher risk of malignant transformation (up to 100% in some long-term studies) compared to those with Ollier disease. In addition to secondary chondrosarcoma, they are at a markedly increased risk for visceral and central nervous system malignancies, most notably astrocytomas, as well as gastrointestinal and ovarian malignancies.

Question 23

A 10-year-old boy with multiple hereditary exostoses (MHE) presents with a progressive forearm deformity. Which of the following is the most characteristic pattern of forearm deformity seen in this condition?

Explanation

Correct Answer: Ulnar shortening, radial bowing, ulnar deviation of the carpus, and radial head dislocation

The classic forearm deformity in Multiple Hereditary Exostoses (MHE) is driven by the disproportionate shortening of the ulna. Because the ulna contributes less to the distal growth plate than the radius, osteochondromas at the distal ulna severely tether its growth. This leads to ulnar shortening, secondary bowing of the radius (which continues to grow but is tethered to the short ulna), ulnar deviation of the carpus (due to lack of ulnar support), and eventual dislocation of the radial head.

Question 24

A 14-year-old girl with polyostotic fibrous dysplasia presents with a progressive 'shepherd's crook' deformity of the proximal femur. Surgical correction with valgus osteotomy is planned. Which of the following grafting techniques is most appropriate to minimize the risk of graft resorption and recurrence?

Explanation

Correct Answer: Cortical allograft

In the surgical management of fibrous dysplasia, particularly for structural deformities like the shepherd's crook deformity of the proximal femur, cortical allograft is the preferred graft material. Cancellous autograft is rapidly resorbed and replaced by the dysplastic fibrous tissue, leading to a high rate of recurrence. Cortical allografts resist resorption much better and provide the necessary structural support while incorporating slowly.

Question 25

A 30-year-old patient presents with severe, deep, aching pain and progressive joint contractures in the lower extremity. Radiographs reveal dense, eccentric, undulating cortical hyperostosis extending along the length of the femur and tibia, resembling 'dripping candle wax'. What is the most likely diagnosis?

Explanation

Correct Answer: Melorheostosis

Melorheostosis is a rare, non-hereditary sclerosing bone dysplasia characterized by linear hyperostosis of the cortex that resembles dripping candle wax on radiographs. It typically affects a single limb in a sclerotomal distribution. Clinically, it presents with pain, joint contractures, and sometimes overlying skin changes (scleroderma-like). It is associated with somatic mutations in the MAP2K1 gene.

Question 26

Which of the following genetic mutations is most strongly associated with the development of Ollier disease?

Explanation

Correct Answer: IDH1

Ollier disease (enchondromatosis) and Maffucci syndrome are associated with somatic mosaic mutations in the isocitrate dehydrogenase 1 (IDH1) or IDH2 genes. These mutations lead to the production of the oncometabolite D-2-hydroxyglutarate, which interferes with normal chondrocyte differentiation. EXT1 is associated with multiple hereditary exostoses, GNAS with fibrous dysplasia, and LEMD3 with osteopoikilosis.

Question 27

An asymptomatic 25-year-old man undergoes a pelvic radiograph following minor trauma, which reveals numerous small, well-defined, symmetric sclerotic foci clustered around the joints. If this skeletal finding is part of a syndrome, what associated skin manifestation might be present?

Explanation

Correct Answer: Connective tissue nevi (dermatofibrosis lenticularis disseminata)

The radiographic description is classic for osteopoikilosis ('spotted bone disease'). When osteopoikilosis is associated with skin lesions, it is known as Buschke-Ollendorff syndrome. The characteristic skin lesions are connective tissue nevi, also known as dermatofibrosis lenticularis disseminata. This autosomal dominant condition is caused by loss-of-function mutations in the LEMD3 gene.

Question 28

A 45-year-old woman with known polyostotic fibrous dysplasia presents with a painless, slow-growing soft tissue mass in her right thigh. MRI shows a well-circumscribed intramuscular lesion that is hyperintense on T2-weighted images and hypointense on T1. Biopsy confirms a benign myxoid neoplasm. What is the eponymous name for this specific syndromic association?

Explanation

Correct Answer: Mazabraud syndrome

Mazabraud syndrome is a rare disorder characterized by the association of fibrous dysplasia (usually polyostotic) and one or more intramuscular myxomas. The myxomas typically occur in the same anatomical region as the most severely affected bones. McCune-Albright syndrome is fibrous dysplasia with endocrinopathies and café-au-lait spots. Jaffe-Campanacci syndrome is the association of multiple non-ossifying fibromas with café-au-lait spots.

Question 29

In patients with multiple hereditary exostoses (MHE), malignant transformation to secondary chondrosarcoma occurs in approximately 1-5% of cases. Which of the following anatomical locations is the most common site for this malignant transformation?

Explanation

Correct Answer: Pelvis

While osteochondromas are most numerous around the knee (distal femur and proximal tibia), malignant transformation to secondary chondrosarcoma in patients with MHE most commonly occurs in the flat bones, particularly the pelvis, followed by the proximal femur and scapula. Lesions in these areas should be monitored closely for growth after skeletal maturity, increasing pain, or a cartilage cap thicker than 1.5 to 2 cm.

Question 30

A 7-year-old girl is diagnosed with McCune-Albright syndrome. In addition to polyostotic fibrous dysplasia and characteristic café-au-lait macules, which of the following is the most common endocrinopathy seen in this condition?

Explanation

Correct Answer: Precocious puberty

McCune-Albright syndrome is defined by the triad of polyostotic fibrous dysplasia, café-au-lait spots (typically with irregular 'Coast of Maine' borders), and autonomous endocrine hyperfunction. The most common endocrinopathy, particularly in females, is gonadotropin-independent precocious puberty. Other possible endocrine abnormalities include hyperthyroidism, growth hormone excess (acromegaly/gigantism), and Cushing syndrome.

Question 31

Which of the following genetic mutations is most commonly shared in the pathogenesis of both Ollier disease and Maffucci syndrome?

Explanation

Correct Answer: C (IDH1)

Ollier disease (multiple enchondromatosis) and Maffucci syndrome (multiple enchondromatosis associated with soft-tissue hemangiomas) are both non-hereditary skeletal dysplasias. Recent genetic studies have identified somatic mosaic mutations in the isocitrate dehydrogenase 1 (IDH1) and IDH2 genes as the primary drivers in both conditions. EXT1 is associated with Multiple Hereditary Exostoses, GNAS1 with Fibrous Dysplasia, LEMD3 with Melorheostosis and Osteopoikilosis, and COMP with Pseudoachondroplasia.

Question 32

A 45-year-old female with a known history of polyostotic fibrous dysplasia presents with a newly noticed, painless, slow-growing soft tissue mass in her right thigh. MRI reveals a well-circumscribed intramuscular lesion with high T2 signal intensity. What is the most likely diagnosis of this soft tissue mass?

Explanation

Correct Answer: B (Intramuscular myxoma)

The association of polyostotic fibrous dysplasia with intramuscular myxomas is known as Mazabraud syndrome. These myxomas are typically benign, painless, and slow-growing. They appear as well-circumscribed masses with high signal intensity on T2-weighted MRI due to their high mucin content. Recognition of this syndrome is important to avoid misdiagnosing the soft tissue mass as a malignant sarcoma.

Question 33

In patients with Multiple Hereditary Exostoses (MHE), the characteristic forearm deformity is primarily driven by osteochondromas affecting which specific location, leading to the classic tethering effect?

Explanation

Correct Answer: D (Distal ulna)

The classic forearm deformity in Multiple Hereditary Exostoses (MHE) is primarily caused by the disproportionate involvement of the distal ulna. The distal ulna has a smaller cross-sectional area and contributes a larger percentage to the overall longitudinal growth of the ulna compared to the radius. Osteochondromas here cause significant growth retardation of the ulna, leading to relative ulnar shortening, secondary bowing of the radius, ulnar deviation of the carpus, and potential radial head dislocation.

Question 34

A 30-year-old male presents with severe, deep, aching pain and progressive joint contracture in his left lower extremity. Radiographs demonstrate dense, irregular, eccentric hyperostosis along the cortex of the femur and tibia, resembling 'dripping candle wax'. Which of the following is the most likely genetic mutation associated with this condition?

Explanation

Correct Answer: B (LEMD3)

The clinical and radiographic description is classic for Melorheostosis, a rare, non-hereditary sclerosing bone dysplasia characterized by 'dripping candle wax' hyperostosis. It is associated with loss-of-function mutations in the LEMD3 gene (also known as MAN1), which is involved in the bone morphogenetic protein (BMP) and TGF-beta signaling pathways. LEMD3 mutations are also associated with Osteopoikilosis and Buschke-Ollendorff syndrome.

Question 35

A 12-year-old boy with polyostotic fibrous dysplasia presents with a progressive 'shepherd's crook' deformity of the proximal femur. Surgical correction is planned. Which of the following principles is most critical for the successful long-term stabilization of this deformity?

Explanation

Correct Answer: C (Stabilization with a load-sharing intramedullary device)

In fibrous dysplasia, the abnormal bone is structurally weak. Bone grafting (both autograft and allograft) typically fails because the graft is resorbed and replaced by dysplastic host bone. Rigid plate fixation also has a high failure rate due to poor screw purchase in the dysplastic bone and stress shielding. The gold standard for stabilizing a corrected 'shepherd's crook' deformity is the use of load-sharing intramedullary devices (like a cephalomedullary nail), which span the entire length of the abnormal bone and protect it from bending forces.

Question 36

An incidental pelvic radiograph of a 25-year-old female reveals numerous small, well-defined, symmetric sclerotic foci clustered around the periarticular regions of the hips and pelvis. She is asymptomatic. If this skeletal finding is part of a syndrome, which of the following dermatological manifestations is most likely to be present?

Explanation

Correct Answer: C (Connective tissue nevi)

The radiographic findings describe Osteopoikilosis ('spotted bone disease'), an asymptomatic, autosomal dominant sclerosing dysplasia. When osteopoikilosis is associated with disseminated connective tissue nevi (dermatofibrosis lenticularis disseminata), the condition is known as Buschke-Ollendorff syndrome. Both isolated osteopoikilosis and Buschke-Ollendorff syndrome are linked to mutations in the LEMD3 gene.

Question 37

A 35-year-old male with a known history of Multiple Hereditary Exostoses (MHE) presents with new-onset pain and enlargement of a previously stable lesion. Which of the following anatomical locations carries the highest risk for malignant transformation of an osteochondroma in this patient population?

Explanation

Correct Answer: C (Pelvis)

Patients with Multiple Hereditary Exostoses (MHE) have an increased risk of malignant transformation to secondary chondrosarcoma (estimated at 1-5%). The risk is significantly higher for lesions located in the axial skeleton and proximal appendicular skeleton, specifically the pelvis, scapula, and proximal femur. Lesions in the distal extremities (like the distal radius or metatarsals) have a very low risk of malignant transformation.

Question 38

A 6-year-old girl is evaluated for a limp and a leg-length discrepancy. Radiographs reveal expansile, ground-glass lesions in her left femur and tibia. Physical examination notes large, hyperpigmented skin macules with irregular borders on her torso. Which of the following endocrine abnormalities is most commonly associated with her likely diagnosis?

Explanation

Correct Answer: B (Precocious puberty)

The patient's presentation of polyostotic fibrous dysplasia and café-au-lait spots with irregular ('coast of Maine') borders is diagnostic of McCune-Albright syndrome. This syndrome is caused by a post-zygotic somatic mutation in the GNAS gene, leading to overactivation of adenylyl cyclase. The most common endocrine abnormality associated with McCune-Albright syndrome is gonadotropin-independent precocious puberty, particularly in females.

Question 39

In a patient with Ollier disease, the characteristic skeletal lesions are primarily a result of which of the following pathophysiological processes?

Explanation

Correct Answer: B (Failure of normal endochondral ossification with persistence of cartilaginous rests)

Ollier disease (multiple enchondromatosis) is characterized by the presence of multiple enchondromas, which are benign hyaline cartilage tumors located within the medullary cavity of bones. Pathophysiologically, these lesions arise from a failure of normal endochondral ossification at the physis, leading to the persistence and proliferation of unossified cartilaginous rests within the metaphysis and diaphysis as the bone grows.

Question 40

A 14-year-old patient undergoes knee radiographs for mild anterior knee pain. The images incidentally reveal multiple, fine, linear radiodense striations running parallel to the long axis of the distal femur and proximal tibia metaphyses. The patient has no other systemic symptoms. What is the most likely diagnosis?

Explanation

Correct Answer: A (Osteopathia striata)

Osteopathia striata (Voorhoeve disease) is a benign, usually asymptomatic sclerosing bone dysplasia characterized radiographically by fine, linear, longitudinal radiodense striations in the metaphyses and diaphyses of long bones. It is often an incidental finding. Osteopoikilosis presents as spotty densities, Melorheostosis as 'flowing wax' cortical hyperostosis, and Engelmann disease (progressive diaphyseal dysplasia) presents with fusiform cortical thickening of the diaphyses.

Question 41

A 12-year-old girl presents with a pathologic fracture of the proximal femur. Radiographs show a 'ground-glass' appearance in the medullary canal with a shepherd's crook deformity. Which of the following genetic mutations is most likely responsible for the underlying condition?

Explanation

Correct Answer: GNAS1 mutation

The clinical and radiographic presentation is classic for fibrous dysplasia. Fibrous dysplasia is caused by a somatic, activating mutation in the GNAS1 gene, which encodes the alpha subunit of the stimulatory G protein (Gs-alpha). This leads to increased intracellular cAMP levels, resulting in abnormal proliferation and differentiation of osteoblasts, replacing normal bone with fibrous stroma and woven bone. FGFR3 mutations are seen in achondroplasia, COMP in pseudoachondroplasia, and EXT1 in multiple hereditary exostoses.

Question 42

A 14-year-old boy presents with multiple asymmetric limb deformities. Radiographs reveal multiple radiolucent lesions in the metaphyses and diaphyses of the long bones and phalanges. Physical examination reveals multiple bluish, compressible soft-tissue masses on the hands and forearms. Which of the following is true regarding his condition compared to patients with isolated multiple enchondromatosis (Ollier disease)?

Explanation

Correct Answer: There is a significantly higher risk of malignant transformation

The patient has Maffucci syndrome, characterized by multiple enchondromas and soft-tissue hemangiomas. Both Ollier disease and Maffucci syndrome are non-hereditary (somatic mutations, often IDH1/IDH2). Maffucci syndrome carries a much higher risk of malignant transformation (up to 100% lifetime risk for any malignancy, including chondrosarcoma, astrocytoma, and GI malignancies) compared to Ollier disease (approximately 25-30% risk of chondrosarcoma).

Question 43

A 10-year-old boy presents with multiple bony prominences around his knees and ankles, causing a valgus deformity at the knees. Family history is positive for similar lesions. The pathogenesis of this condition involves a defect in which of the following?

Explanation

Correct Answer: Heparan sulfate glycosaminoglycan synthesis

The patient has Multiple Hereditary Exostoses (MHE), also known as diaphyseal aclasis. MHE is an autosomal dominant condition caused by mutations in the EXT1 or EXT2 genes. These genes encode glycosyltransferases that are essential for the synthesis of heparan sulfate, a glycosaminoglycan. The deficiency in heparan sulfate disrupts normal Indian hedgehog (Ihh) signaling in the growth plate, leading to the formation of osteochondromas.

Question 44

A 35-year-old woman presents with chronic, deep aching pain in her right lower extremity and decreased range of motion in her knee. Radiographs reveal dense, irregular, eccentric hyperostosis along the cortex of the femur and tibia, resembling 'dripping candle wax'. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Melorheostosis

Melorheostosis is a rare, non-hereditary sclerosing bone dysplasia characterized by linear hyperostosis of the cortex, classically described as having a 'dripping candle wax' appearance on radiographs. It typically affects one limb (monomelic) and can cause severe pain, joint contractures, and soft tissue fibrosis. Osteopoikilosis presents as multiple small sclerotic foci ('spotted bone'), while osteopathia striata presents as linear striations in the metaphyses.

Question 45

An asymptomatic 25-year-old man undergoes a routine pelvic radiograph following a minor trauma. The radiograph incidentally shows numerous small, well-defined, symmetric, periarticular sclerotic foci in the pelvis and proximal femurs. If this patient has an associated skin condition, what is the most likely dermatological finding?

Explanation

Correct Answer: Dermatofibrosis lenticularis disseminata

The radiographic findings are classic for osteopoikilosis ('spotted bone disease'), an autosomal dominant sclerosing dysplasia caused by LEMD3 mutations. When osteopoikilosis is associated with connective tissue nevi (specifically dermatofibrosis lenticularis disseminata), the condition is known as Buschke-Ollendorff syndrome. Café-au-lait spots are seen in Neurofibromatosis and McCune-Albright syndrome. Hemangiomas are seen in Maffucci syndrome.

Question 46

A 14-year-old boy with polyostotic fibrous dysplasia presents with progressive varus bowing of the proximal femur (shepherd's crook deformity) and increasing thigh pain. What is the most appropriate surgical management for this deformity?

Explanation

Correct Answer: Valgus producing osteotomy and intramedullary nailing

In fibrous dysplasia, the entire bone is often mechanically compromised. Bone grafts (especially cancellous) are typically resorbed and replaced by dysplastic host bone, leading to high failure rates. Plate fixation is also prone to failure due to poor screw purchase in the dysplastic bone and the stress-shielding effect. The gold standard for correcting a shepherd's crook deformity is a valgus-producing osteotomy stabilized with a load-sharing intramedullary device (nail) that spans the entire length of the abnormal bone.

Question 47

A 45-year-old female with a known history of polyostotic fibrous dysplasia presents with a painless, slow-growing soft tissue mass in her right thigh. Magnetic resonance imaging (MRI) reveals a well-circumscribed, intramuscular cystic-appearing lesion with low T1 and high T2 signal intensity. What is the most likely diagnosis of the soft tissue mass?

Explanation

Correct Answer: Intramuscular myxoma

The association of fibrous dysplasia with intramuscular myxomas is known as Mazabraud syndrome. These myxomas are benign, slow-growing soft tissue tumors that typically appear in the same anatomical region as the most severely affected bones. On MRI, they appear cystic (low T1, high T2) but enhance with gadolinium. Recognition of this syndrome prevents unnecessary aggressive biopsies or resections for suspected malignancy.

Question 48

A 28-year-old man with a history of multiple hereditary exostoses presents with a new onset of pain and enlargement of a previously stable lesion on his proximal humerus. MRI is performed. Which of the following MRI findings is most concerning for malignant transformation to secondary chondrosarcoma?

Explanation

Correct Answer: Cartilage cap thickness of 2.5 cm

Patients with multiple hereditary exostoses have a 1-5% lifetime risk of malignant transformation to secondary chondrosarcoma. Clinical signs include new pain or growth after skeletal maturity. On MRI, a cartilage cap thickness greater than 1.5 to 2.0 cm in an adult is highly suspicious for malignant transformation. Continuity of the marrow cavity is a defining feature of all osteochondromas, benign or malignant.

Question 49

A 10-year-old girl is evaluated for mild developmental delay and macrocephaly. Radiographs of her lower extremities reveal multiple, parallel, linear bands of sclerosis extending from the metaphyses into the diaphyses of the long bones. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Osteopathia striata

Osteopathia striata (Voorhoeve disease) is a rare sclerosing bone dysplasia characterized radiographically by linear, parallel striations of dense bone in the metaphyses and diaphyses of long bones, and a sunburst appearance in the ilium. It can occur isolated or as part of osteopathia striata with cranial sclerosis (OSCS), an X-linked dominant condition caused by WTX gene mutations, which presents with macrocephaly, cleft palate, and developmental delay.

Question 50

A 7-year-old girl presents with a history of early breast development and vaginal bleeding. Physical examination reveals large, irregular café-au-lait macules with 'coast of Maine' borders on her trunk. Radiographs show expansile, ground-glass lesions in her left femur and tibia. Which of the following endocrine abnormalities is most commonly associated with this patient's syndrome?

Explanation

Correct Answer: Precocious puberty

The patient has McCune-Albright syndrome, a classic triad of polyostotic fibrous dysplasia, café-au-lait spots (with irregular 'coast of Maine' borders), and endocrine hyperfunction. The most common endocrine abnormality is gonadotropin-independent precocious puberty, especially in females. Other possible endocrine issues include hyperthyroidism, growth hormone excess, and Cushing syndrome.

Question 51

A 6-month-old infant with a known skeletal dysplasia presents with central sleep apnea, hyperreflexia, and clonus. Radiographs show a narrow interpedicular distance in the lumbar spine. Which molecular mechanism primarily drives the underlying condition?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3, which inhibits chondrocyte proliferation in the growth plate. Foramen magnum stenosis is a critical complication in infancy that can cause cervicomedullary compression and central sleep apnea.

Question 52

A 5-year-old child presents with a "hitchhiker" thumb, bilateral rigid clubfeet, and cauliflower ears. Radiographs reveal short, thick tubular bones. Which of the following genes is most likely mutated in this patient?

Explanation

Diastrophic dysplasia is an autosomal recessive condition caused by a defect in the SLC26A2 gene, resulting in impaired sulfate transport. Classic features include hitchhiker thumbs, cauliflower ears, and rigid clubfeet.

Question 53

An 8-year-old boy presents with progressive severe thoracolumbar kyphoscoliosis, a short trunk, and normal intelligence. He has a history of bilateral retinal detachments. Which of the following cervical spine abnormalities is most critical to screen for in this patient?

Explanation

Spondyloepiphyseal dysplasia congenita (SEDC) is caused by a COL2A1 mutation. Patients present with short-trunk dwarfism and myopia or retinal detachment, and they are at high risk for atlantoaxial instability secondary to odontoid hypoplasia.

Question 54

A 7-year-old girl with normal intelligence and no facial dysmorphism presents with a waddling gait. Radiographs show delayed ossification of the capital femoral epiphyses and fragmented epiphyses in multiple joints. Genetic testing reveals a mutation in the COMP gene. What is the diagnosis?

Explanation

Pseudoachondroplasia is primarily caused by a mutation in the Cartilage Oligomeric Matrix Protein (COMP) gene. Unlike achondroplasia, these patients have normal facial features and their skeletal changes become clinically apparent after the first year of life.

Question 55

A 3-year-old is being evaluated for short stature, frontal bossing, and delayed closure of cranial sutures. The clavicles are completely absent on chest radiographs. Which transcription factor is primarily affected in this disorder?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) transcription factor. RUNX2 is critical for osteoblast differentiation; its deficiency leads to absent clavicles and delayed cranial suture closure.

Question 56

A neonate presents with multiple fractures, blue sclerae, and profound osteopenia. The parents are carriers of a severe recessive form of this disease. Which of the following best describes the fundamental defect in the most common dominant forms of this condition?

Explanation

Osteogenesis Imperfecta is most commonly caused by mutations in COL1A1 or COL1A2, leading to the substitution of glycine by a bulkier amino acid. This fundamentally disrupts the formation and stability of the type I collagen triple helix.

Question 57

A 4-year-old boy presents with knock-knees and short stature. Labs show normal serum calcium, normal phosphorus, normal alkaline phosphatase, and normal vitamin D levels. Radiographs show coxa vara, bowing of the femurs, and flared, irregular metaphyses. What is the most likely diagnosis?

Explanation

Schmid type metaphyseal chondrodysplasia (caused by a COL10A1 mutation) mimics rickets clinically and radiographically. However, all serum biochemical markers (calcium, phosphorus, alkaline phosphatase) are strictly normal.

Question 58

A newborn is diagnosed with an autosomal recessive condition characterized by severe osteopetrosis, recurrent infections, and cranial nerve palsies. The underlying genetic defect most likely disrupts which of the following cellular components?

Explanation

Malignant infantile osteopetrosis is typically caused by mutations affecting osteoclast function, such as TCIRG1 (proton pump) or ClCN7 (chloride channel). Failure to acidify the resorption pit leads to unresorbed primary spongiosa and bone marrow obliteration.

Question 59

An 18-month-old girl presents with short stature, severe hypercalcemia, and hypophosphatemia. Radiographs show widespread metaphyseal cupping and fraying with striking osteopenia. Genetic testing reveals a mutation in the PTHR1 gene. What is the diagnosis?

Explanation

Jansen metaphyseal chondrodysplasia is caused by a gain-of-function mutation in the PTH/PTHrP receptor (PTHR1). This leads to ligand-independent activation, mimicking severe hyperparathyroidism with profound hypercalcemia.

Question 60

A 12-year-old boy presents with recurrent spontaneous fractures. He is notably short, has dysmorphic facies with a prominent forehead, and exhibits acro-osteolysis of the distal phalanges. Radiographs demonstrate dense bones but open cranial sutures. Which enzyme is deficient in this patient?

Explanation

Pycnodysostosis is an autosomal recessive lysosomal storage disease caused by a deficiency in Cathepsin K. Classic features include osteosclerosis, acro-osteolysis, delayed cranial suture closure, and recurrent fractures.

Question 61

A 6-year-old child presents with extreme ligamentous laxity, short stature, and progressive severe kyphoscoliosis. Urine analysis reveals elevated levels of keratan sulfate. Radiographs demonstrate universal platyspondyly with anterior central beaking. Which of the following is an absolute surgical indication in this disorder?

Explanation

Morquio syndrome (MPS IV) is characterized by keratan sulfate accumulation and extreme ligamentous laxity. Odontoid hypoplasia with atlantoaxial instability is a hallmark; any associated neurologic deficit warrants urgent cervical spine fusion.

Question 62

A 9-year-old girl is evaluated for a significant leg length discrepancy. Physical exam reveals a unilateral port-wine stain over her hypertrophic lower extremity. Superficial varicosities are prominently noted on the same limb. Which genetic mutation is primarily associated with this syndrome?

Explanation

Klippel-Trenaunay syndrome consists of the triad of capillary malformations (port-wine stain), venous malformations, and limb overgrowth. It is associated with somatic activating mutations in the PIK3CA gene.

Question 63

A 30-year-old female presents with bilateral knee pain. Radiographs reveal a "double-layer" patella on the lateral view and flattened irregular femoral condyles. She has normal height and normal facial features. Which of the following conditions does she most likely have?

Explanation

Multiple Epiphyseal Dysplasia (MED) can be caused by mutations in MATN3, COMP, or COL9. The "double-layer" patella is a classic radiographic sign uniquely seen in the Fairbank type of MED.

Question 64

A neonate presents with short limbs, postaxial polydactyly, severely hypoplastic nails, and a cardiac murmur. An echocardiogram confirms the presence of a common atrium. Which of the following diagnoses is most likely?

Explanation

Ellis-van Creveld syndrome (chondroectodermal dysplasia) is caused by EVC gene mutations. It distinctively features short-limb dwarfism, polydactyly, dysplastic nails, and congenital heart defects, most commonly an ASD or single atrium.

Question 65

A 14-year-old boy with Neurofibromatosis Type 1 (NF1) is evaluated for a rapidly progressive spinal deformity. Radiographs show a short, sharp, acutely angulated curve spanning four contiguous vertebrae with severe vertebral wedging and rib penciling. What is the most appropriate management for this specific spinal deformity?

Explanation

Dystrophic scoliosis in NF1 is highly progressive and carries a significant risk of pseudoarthrosis. Standard treatment requires rigid combined anterior and posterior spinal arthrodesis.

Question 66

A 6-month-old male with achondroplasia presents for a routine check-up. The parents report recent episodes of witnessed sleep apnea, profound hypotonia, and poor feeding. Which of the following is the most appropriate next step in management?

Explanation

Infants with achondroplasia are at high risk for foramen magnum stenosis, leading to cervicomedullary compression, central sleep apnea, and sudden death. MRI of the craniocervical junction is the gold standard for evaluating this potentially lethal complication.

Question 67

A 4-year-old girl is evaluated for short stature and a waddling gait. Examination reveals normal facial features, marked joint laxity, and disproportionate short-limb dwarfism. Radiographs show delayed epiphyseal ossification and irregular, fragmented metaphyses. A mutation in which of the following genes is most likely responsible?

Explanation

Pseudoachondroplasia is caused by mutations in the COMP (Cartilage Oligomeric Matrix Protein) gene. Unlike achondroplasia, patients have normal craniofacial features and head circumference, but present with joint laxity, early-onset osteoarthritis, and short-limb dwarfism.

Question 68

A newborn presents with micromelic severe short stature, rigid equinovarus foot deformities, and bilaterally abducted "hitchhiker" thumbs. Physical examination also reveals cystic swelling of the external ear. What is the underlying genetic defect in this condition?

Explanation

Diastrophic dysplasia is an autosomal recessive condition caused by mutations in the SLC26A2 (DTDST) gene, which encodes a sulfate transporter. This leads to under-sulfated cartilage proteoglycans, classically presenting with hitchhiker thumbs, cauliflower ears, and rigid clubfeet.

Question 69

A 9-year-old boy complains of bilateral knee pain and a waddling gait. Radiographs show small, irregular, and fragmented epiphyses, but normal metaphyses and a normal spine. Lateral radiographs of the knee reveal a "double-layer" appearance of the patella. Which of the following conditions is most likely?

Explanation

Multiple Epiphyseal Dysplasia (MED) primarily affects the epiphyses, presenting with early-onset joint pain and a waddling gait. The "double-layer" patella on a lateral radiograph is a pathognomonic finding for this condition, distinguishing it from other dysplasias.

Question 70

A 6-year-old boy with disproportionate short-trunk dwarfism undergoes preoperative evaluation for severe coxa vara. He has a history of high myopia and a prior retinal detachment. Radiographs reveal flattening of the vertebral bodies. What crucial evaluation must be performed before administering general anesthesia?

Explanation

Spondyloepiphyseal Dysplasia Congenita (SEDC) is a type II collagenopathy (COL2A1) associated with odontoid hypoplasia and atlantoaxial instability. Flexion-extension cervical spine radiographs are essential before surgery to prevent spinal cord injury during intubation.

Question 71

A 10-year-old boy presents to the orthopedic clinic with a wide waddling gait and the unusual ability to appose his shoulders anteriorly across his chest. Radiographs show widening of the symphysis pubis and coxa vara. Which transcription factor is primarily mutated in this syndrome?

Explanation

Cleidocranial dysplasia is an autosomal dominant disorder caused by a mutation in the RUNX2 (CBFA1) gene, which is essential for osteoblast differentiation. Patients classically exhibit absent or hypoplastic clavicles, delayed cranial suture closure, and wide pubic symphysis.

Question 72

An infant presents with failure to thrive, hepatosplenomegaly, and severe anemia. Radiographs demonstrate diffusely dense bones with a "bone-within-a-bone" appearance and loss of medullary canals. Which of the following represents the primary cellular mechanism defective in this severe condition?

Explanation

Malignant infantile osteopetrosis is caused by defective osteoclast function, most commonly due to mutations affecting the ruffled border (e.g., TCIRG1). The inability to resorb bone leads to dense but brittle bones, marrow obliteration, and extramedullary hematopoiesis.

Question 73

A 30-year-old man presents with a transverse fracture of the femoral shaft. He has a history of multiple prior fractures, short stature, and prominent eyes. Radiographs reveal generalized osteosclerosis, acro-osteolysis of the distal phalanges, and an absent mandibular angle. What is the mutated enzyme responsible for this disorder?

Explanation

Pycnodysostosis is an autosomal recessive disorder caused by a mutation in the Cathepsin K gene, an enzyme crucial for degrading type I collagen during bone resorption. It is uniquely characterized by osteosclerosis, delayed suture closure, absent mandibular angle, and acro-osteolysis.

Question 74

A newborn presents with severe respiratory distress, shortened and anteriorly bowed lower extremities, and skin dimpling over the tibial bows. Karyotype analysis reveals a 46,XY genotype, but the external genitalia appear female. A mutation in which of the following genes is the definitive cause?

Explanation

Campomelic dysplasia is caused by a heterozygous mutation in the SOX9 gene, a transcription factor essential for chondrogenesis and sex determination. The syndrome is characterized by anterior bowing of the tibiae, respiratory failure due to tracheobronchomalacia, and XY sex reversal.

Question 75

A 4-year-old child presents with painful, warm, soft tissue swellings on his back following minor trauma. Physical examination notes bilateral congenital hallux valgus and microdactyly of the first toes. Biopsy of the back lesions is deferred due to the suspicion of a specific genetic condition. What is the pathogenic mutation involved?

Explanation

Fibrodysplasia Ossificans Progressiva (FOP) is caused by an activating mutation in the ACVR1 (ALK2) gene, a BMP type I receptor. It is characterized by congenital malformation of the great toes and progressive heterotopic ossification, which is catastrophically exacerbated by trauma or surgery.

Question 76

A 12-year-old boy with a history of recurrent fractures is diagnosed with Osteogenesis Imperfecta (OI) Type V. Which of the following radiographic or clinical findings is uniquely characteristic of this specific type of OI compared to classic forms?

Explanation

Osteogenesis Imperfecta Type V is an autosomal dominant non-collagenous form of OI caused by an IFITM5 mutation. It is classically distinguished from other types by the triad of hyperplastic callus formation following fractures, radioulnar synostosis, and an ossified interosseous membrane.

Question 77

A 4-year-old boy with achondroplasia presents with delayed motor milestones, hypotonia, and central sleep apnea. What is the most appropriate initial screening study for this patient's suspected complication?

Explanation

Cervicomedullary compression at the foramen magnum is a life-threatening complication of achondroplasia, presenting with sleep apnea and hypotonia. An MRI of the craniocervical junction is essential to evaluate for stenosis and cord signal changes.

Question 78

A newborn presents with short-limbed dwarfism, bilateral clubfeet, hitchhiker thumbs, and cystic swelling of the pinnae. Radiographs reveal a severe mid-cervical kyphosis. What is the typical natural history of this cervical deformity?

Explanation

Diastrophic dysplasia is caused by a sulfate transporter mutation (SLC26A2). Unlike the progressive cervical kyphosis seen in other conditions, the mid-cervical kyphosis in diastrophic dysplasia often resolves spontaneously with growth.

Question 79

A 6-year-old girl is evaluated for disproportionate short stature, waddling gait, and joint laxity. Her facial features are entirely normal. Radiographs demonstrate fragmented, irregular epiphyses and metaphyseal flaring. A mutation in which of the following genes is most likely responsible?

Explanation

Pseudoachondroplasia is caused by a mutation in the COMP (Cartilage Oligomeric Matrix Protein) gene. It classically presents with disproportionate short stature and joint laxity but spares the craniofacial skeleton, distinguishing it from achondroplasia.

Question 80

A 7-year-old child with a known mutation in the COL2A1 gene requires bilateral femoral osteotomies for severe coxa vara. Associated clinical findings include a barrel chest, myopia, and flat midface. Prior to intubation, which of the following must be evaluated?

Explanation

Spondyloepiphyseal dysplasia congenita (SEDC) is a type II collagenopathy commonly associated with odontoid hypoplasia. Flexion-extension radiographs of the cervical spine are mandatory before anesthesia to rule out atlantoaxial instability.

Question 81

A 15-year-old male with delayed closure of cranial sutures and supernumerary teeth is evaluated for a unilateral limp. Examination reveals significant shoulder hypermobility. Which of the following hip abnormalities is most commonly associated with his condition?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a RUNX2 (CBFA1) mutation. Skeletal manifestations typically include absent or hypoplastic clavicles, delayed skull ossification, and developmental coxa vara.

Question 82

A 9-year-old boy presents with severe knock-knees, platyspondyly, and corneal clouding. Laboratory tests reveal increased urinary excretion of keratan sulfate. This patient's condition is an autosomal recessive lysosomal storage disease caused by a deficiency in which enzyme?

Explanation

Morquio A syndrome (MPS IVA) is caused by a deficiency of N-acetylgalactosamine-6-sulfatase, leading to keratan sulfate accumulation. Patients typically present with severe skeletal dysplasia and atlantoaxial instability, but maintain normal intelligence.

Question 83

A 5-year-old girl is brought to the clinic due to painful swelling of her neck and back. Examination reveals a shortened great toe with hallux valgus on both feet. Hard, subcutaneous masses are palpated along her paraspinal muscles. What is the most critical management principle for this patient?

Explanation

Fibrodysplasia ossificans progressiva (FOP) is characterized by an ACVR1 mutation and classic great toe malformations. Any minor trauma, including biopsies, intramuscular injections, or surgery, triggers explosive heterotopic ossification and must be strictly avoided.

Question 84

A 10-year-old boy with blue sclerae, dentinogenesis imperfecta, and multiple prior fractures is scheduled for rodding of his tibiae. His condition is caused by a dominant mutation in COL1A1. Which of the following best describes the fundamental defect in his bone collagen?

Explanation

Osteogenesis imperfecta is primarily caused by mutations in COL1A1 or COL1A2. The classic qualitative defect involves the substitution of glycine, the smallest amino acid, with a bulkier residue, disrupting the stable formation of the collagen triple helix.

Question 85

A 28-year-old female with disproportionate short stature reports progressive, bilateral knee pain. She has a history of mild short-limbed dwarfism but normal spine radiographs. Lateral knee radiographs reveal a distinctive bony anomaly. Which of the following is the pathognomonic finding for her likely condition?

Explanation

Multiple Epiphyseal Dysplasia (MED) can be caused by mutations in COMP, MATN3, or type IX collagen genes. A double-layer patella visualized on the lateral radiograph is a highly characteristic and pathognomonic finding for MED.

Question 86

A 4-year-old boy presents with short stature, waddling gait, and progressive bowing of the lower extremities. Laboratory studies demonstrate normal calcium, phosphorus, and alkaline phosphatase levels. Radiographs show coxa vara and flared, irregular metaphyses, but the epiphyses are normal. A defect in which collagen type is most likely responsible?

Explanation

Schmid type metaphyseal chondrodysplasia clinically and radiographically resembles rickets but has normal serologic profiles. It is caused by a mutation in the COL10A1 gene, resulting in abnormal type X collagen within the hypertrophic zone of the physis.

Question 87

A newborn presents with rhizomelic shortening of the extremities, frontal bossing, and midface hypoplasia. What is the specific genetic mutation and its cellular effect on the physeal growth plate in this condition?

Explanation

Achondroplasia is caused by an autosomal dominant gain-of-function mutation in the FGFR3 gene. This constitutive activation paradoxically inhibits chondrocyte proliferation in the proliferative zone of the physis, leading to diminished endochondral ossification.

Question 88

A 5-year-old child presents with recurrent fractures, severe anemia, and hepatosplenomegaly. Radiographs exhibit a generalized increased bone density with a classic "bone-within-bone" appearance. What is the primary cellular defect responsible for this condition?

Explanation

Malignant infantile osteopetrosis is commonly caused by mutations in genes such as TCIRG1 or carbonic anhydrase II. This leads to an inability of osteoclasts to acidify Howship's lacunae, preventing normal bone resorption and leading to dense, brittle bones with marrow obliteration.

Question 89

A newborn presents with severe micromelic limb shortening, rigid clubfeet, "hitchhiker" thumbs, and swollen pinnae. What is the affected gene and the expected inheritance pattern for this dysplasia?

Explanation

Diastrophic dysplasia is an autosomal recessive disorder caused by mutations in the SLC26A2 (DTDST) gene, which impairs proper cellular sulfate transport. Classical features include the hitchhiker thumb deformity, cauliflower ears, and severe, rigid clubfeet.

Question 90

A 6-year-old child with a short trunk, coxa vara, and high myopia presents with progressive weakness and hyperreflexia in all four extremities. What is the most critical cervical spine anomaly that must be evaluated in this patient?

Explanation

Spondyloepiphyseal Dysplasia Congenita (SEDC) is a type II collagenopathy (COL2A1) characterized by short-trunk dwarfism. Odontoid hypoplasia is extremely common and can lead to severe atlantoaxial instability, requiring careful screening with flexion-extension cervical radiographs.

Question 91

A 10-year-old boy presents to the orthopedic clinic with the ability to touch his shoulders together anteriorly across his chest. Examination reveals delayed closure of the fontanelles and multiple supernumerary teeth. Which transcription factor is mutated in this condition?

Explanation

Cleidocranial dysplasia is an autosomal dominant disorder caused by mutations in the CBFA1 (RUNX2) gene, an essential transcription factor for osteoblast differentiation. It is characterized by hypoplastic or absent clavicles, delayed cranial suture closure, and dental anomalies.

Question 92

A child appeared normal at birth but develops severe short-limbed dwarfism by age 3, with normal facial features, joint laxity, and early-onset osteoarthritis. Accumulation of which abnormal protein in the rough endoplasmic reticulum of chondrocytes is responsible?

Explanation

Pseudoachondroplasia is caused by mutations in the COMP gene. This leads to the accumulation of misfolded COMP in the rough endoplasmic reticulum of chondrocytes, causing premature chondrocyte death and resulting in delayed-onset dwarfism with normal facies.

Question 93

A 5-year-old boy presents with normal intelligence, corneal clouding, genu valgum, and a prominent pectus carinatum. Urine spot tests show elevated keratan sulfate. Deficiency of which of the following enzymes is the cause of this syndrome?

Explanation

Morquio syndrome (Mucopolysaccharidosis Type IV) is caused by a deficiency in N-acetylgalactosamine-6-sulfatase (Type A) or beta-galactosidase (Type B). It leads to keratan sulfate accumulation, severe skeletal deformities, and cervical instability, notably without intellectual impairment.

Question 94

An infant is born with multiple severe fractures, deeply blue sclerae, and profound limb deformity. The child expires shortly after birth due to respiratory failure. According to the Sillence classification, which type of Osteogenesis Imperfecta does this represent?

Explanation

Sillence Type II is the perinatal lethal form of Osteogenesis Imperfecta. It is characterized by severe osseous fragility, crumpled long bones, and beaded ribs, with death usually occurring secondary to respiratory failure from restricted thoracic volume.

Question 95

A 12-year-old boy presents with bilateral knee and hip pain, mild short stature, and a waddling gait. Radiographs show delayed, fragmented ossification centers of the femoral heads bilaterally, but normal spinal alignment and vertebral body heights. Which condition must this most carefully be differentiated from?

Explanation

Multiple Epiphyseal Dysplasia (MED) often presents with bilateral, symmetric hip involvement that closely mimics bilateral Legg-Calvé-Perthes (LCP) disease. Normal spine radiographs (lack of platyspondyly) help distinguish MED from spondyloepiphyseal dysplasia (SED).

Question 96

A child presents with short-trunk dwarfism, a flat midface, cleft palate, and prominent, stiff joints. Radiographs demonstrate coronal clefts of the vertebral bodies and epiphyseal stippling with a classic "dumbbell" appearance of the long bones. What is the most likely diagnosis?

Explanation

Kniest dysplasia is a type II collagenopathy characterized by dumbbell-shaped femora, coronal clefts in the vertebral bodies, and enlarged, stiff joints. Cleft palate and hearing loss are also highly characteristic clinical findings.

Question 97

A newborn with phenotypic female genitalia is found to have a 46,XY karyotype. The infant exhibits marked anterolateral bowing of the tibiae and femora, hypoplastic scapulae, and severe respiratory distress. A mutation in which gene is responsible for this condition?

Explanation

Campomelic dysplasia is an autosomal dominant condition caused by mutations in the SOX9 gene, leading to severe bowing of long bones and fatal respiratory distress from tracheomalacia. Because SOX9 is crucial for sex determination, XY individuals frequently present with sex reversal.

Question 98

A 30-year-old patient has routine radiographs following minor trauma that reveal numerous small, symmetric, well-defined sclerotic bone lesions clustered around the periarticular regions of the pelvis and long bones. What associated skin finding is most commonly seen with this condition?

Explanation

Osteopoikilosis (spotted bone disease) is an asymptomatic sclerosing bone dysplasia associated with LEMD3 mutations. It is classically associated with dermatofibrosis lenticularis disseminata (Buschke-Ollendorff syndrome), presenting as connective tissue nevi.

Question 99

A 4-year-old child presents with short stature, severe bowing of the legs, and a waddling gait. Radiographs demonstrate flaring and irregularity of the metaphyses with widened physes mimicking rickets, but serum calcium and phosphorus are normal. Mutation in the gene encoding which collagen type is responsible?

Explanation

Schmid type metaphyseal chondrodysplasia is caused by mutations in the COL10A1 gene, encoding type X collagen. Type X collagen is exclusively expressed by hypertrophic chondrocytes in the growth plate, leading to the specific metaphyseal irregularities that mimic rickets.

Question 100

A 15-year-old boy with polyostotic fibrous dysplasia presents with a progressive "shepherd's crook" deformity of the proximal femur and mechanical hip pain. Which surgical construct is most appropriate for stabilizing the femur following corrective osteotomy?

Explanation

In fibrous dysplasia, the weak fibro-osseous tissue makes extramedullary plates prone to failure due to poor screw purchase. Intramedullary nailing is the preferred fixation method as it protects the entire length of the bone and effectively unloads the dysplastic cortex.

Pediatric Orthopaedics: Comprehensive MCQ Question Bank & Exam Prep

Skeletal Dysplasias: Anarchic Bone Development MCQs

Skeletal Dysplasias: Anarchic Bone Development MCQs

Chapter Index