Skeletal Dysplasias & Anarchic Bone Development MCQs

Question 1

A 12-year-old male presents with multiple painless bony prominences around the knees and ankles. Genetic testing reveals a mutation in the EXT1 gene. Which of the following is the most likely mechanism of inheritance for this condition?

Explanation

Correct Answer: B

Hereditary Multiple Exostoses (HME), also known as diaphyseal aclasis, is inherited in an Autosomal Dominant fashion. It is primarily associated with mutations in the EXT1 (Chromosome 8) and EXT2 (Chromosome 11) genes, which encode glycosyltransferases involved in heparan sulfate synthesis.

Question 2

In a patient diagnosed with Hereditary Multiple Exostoses (HME), which clinical finding is considered a 'red flag' for malignant transformation into secondary chondrosarcoma after skeletal maturity?

Explanation

Correct Answer: C

Malignant transformation in HME occurs in approximately 1-5% of cases. Signs of malignancy include new onset of pain after skeletal maturity, rapid growth, and a cartilage cap thickness exceeding 1.5 to 2 cm in adults. Lesions pointing away from the joint are typical of benign osteochondromas.

Question 3

A 22-year-old female presents with multiple enchondromas localized primarily to the left side of her body, including the hand and femur. There are no associated soft tissue hemangiomas. What is the most likely diagnosis?

Explanation

Correct Answer: C

Ollier's disease (Enchondromatosis) is characterized by multiple enchondromas, often with a unilateral or asymmetrical distribution. Unlike Maffucci syndrome, it does not involve soft tissue hemangiomas. It is a non-hereditary condition resulting from somatic mutations.

Question 4

Which of the following conditions carries the highest lifetime risk (approaching 100% in some series) for the development of both skeletal and extra-skeletal malignancies, including visceral carcinomas?

Explanation

Correct Answer: C

Maffucci syndrome (Enchondromatosis plus hemangiomas) carries a significantly higher risk of malignancy than Ollier's disease. Patients have a very high risk of chondrosarcoma and are also predisposed to visceral malignancies such as gliomas and ovarian carcinomas.

Question 5

A 9-year-old girl presents with a 'Shepherd's crook' deformity of the proximal femur and large, irregular café-au-lait spots with 'Coast of Maine' borders. Which molecular defect is responsible for this condition?

Explanation

Correct Answer: B

The patient has McCune-Albright syndrome (Polyostotic Fibrous Dysplasia, café-au-lait spots, and precocious puberty). This is caused by a post-zygotic somatic mutation in the GNAS gene, leading to constitutive activation of the alpha subunit of the stimulatory G-protein (Gs-alpha) and increased intracellular cAMP.

Question 6

Radiographic evaluation of a 15-year-old's femur shows a well-circumscribed intramedullary lesion with a 'ground-glass' appearance and a thin sclerotic rim (rind sign). What is the most appropriate initial management for an asymptomatic monostotic lesion in this location?

Explanation

Correct Answer: C

Asymptomatic monostotic Fibrous Dysplasia (FD) often does not require surgical intervention and can be managed with observation. Surgery (curettage, grafting, or stabilization) is reserved for lesions that are symptomatic, causing significant deformity, or at high risk of pathological fracture.

Question 7

A patient with polyostotic fibrous dysplasia is found to have multiple soft tissue myxomas. This clinical association is known as:

Explanation

Correct Answer: B

Mazabraud syndrome is the rare association of fibrous dysplasia (usually polyostotic) and multiple soft tissue myxomas. These patients require monitoring for both skeletal complications and the potential for malignant transformation of the FD lesions.

Question 8

Dysplasia Epiphysealis Hemimelica (Trevor's Disease) is histologically indistinguishable from which of the following tumors?

Explanation

Correct Answer: C

Trevor's disease is essentially an osteochondroma arising from the epiphysis (epiphyseal overgrowth). Histologically, it shows a benign osteocartilaginous cap similar to a standard osteochondroma, but its location is intra-articular or peri-articular at the epiphysis.

Question 9

In the surgical management of a 'Shepherd's crook' deformity in a patient with Fibrous Dysplasia, which of the following is the preferred method of fixation to prevent recurrence of the deformity?

Explanation

Correct Answer: C

For the Shepherd's crook deformity in Fibrous Dysplasia, intramedullary fixation is preferred over plates or bone grafts alone. Intramedullary devices provide better load-sharing and support the entire weakened femoral shaft, reducing the high rate of deformity recurrence seen with other methods.

Question 10

A 7-year-old child presents with asymmetrical overgrowth of the medial aspect of the distal femoral epiphysis, causing a progressive valgus deformity. The lesion is osteocartilaginous. What is the most likely diagnosis?

Explanation

Correct Answer: C

Dysplasia Epiphysealis Hemimelica (Trevor's disease) typically presents as an asymmetrical overgrowth of an epiphysis, most commonly in the lower limb (talus, knee, or hip). It leads to joint deformity, restricted motion, and mechanical symptoms.

Question 11

A 12-year-old male presents with multiple painless bony prominences around the knees and ankles. Genetic testing reveals a mutation in the EXT1 gene. What is the primary biochemical defect associated with this condition?

Explanation

Correct Answer: B

Multiple Hereditary Exostoses (MHE) is caused by mutations in EXT1 or EXT2 genes. These genes encode glycosyltransferases involved in the synthesis of heparan sulfate (HS) glycosaminoglycans. A deficiency in HS leads to abnormal chondrocyte proliferation and maturation in the growth plate, resulting in exostosis formation.

Question 12

In a patient with Multiple Hereditary Exostoses (MHE), which of the following forearm deformities is most characteristic of the 'Bessel-Hagen' deformity?

Explanation

Correct Answer: B

The Bessel-Hagen deformity in MHE is characterized by disproportionate shortening of the ulna (due to its smaller diameter and greater relative growth plate involvement), which leads to secondary bowing of the radius, ulnar tilt of the distal radial articular surface, and often, proximal translocation (dislocation) of the radial head.

Question 13

A 25-year-old female with known Ollier disease presents with new-onset deep-seated pain in her proximal humerus. Radiographs show an enlarging lucent lesion with endosteal scalloping. What is the estimated lifetime risk of malignant transformation to chondrosarcoma in patients with Ollier disease?

Explanation

Correct Answer: C

While solitary enchondromas have a very low risk of malignant transformation (<1%), patients with Enchondromatosis (Ollier disease) have a significantly higher risk, often cited between 25% and 40%. In Maffucci syndrome, the risk of skeletal and extra-skeletal malignancy is even higher.

Question 14

Maffucci syndrome is distinguished from Ollier disease by the presence of which of the following clinical features?

Explanation

Correct Answer: B

Maffucci syndrome is characterized by multiple enchondromas (similar to Ollier disease) plus soft tissue hemangiomas (often spindle-cell hemangiomas). Both conditions are non-hereditary and result from somatic mutations (IDH1 or IDH2).

Question 15

A 9-year-old girl presents with a 'ground-glass' lesion in the femoral neck and large, irregular café-au-lait spots with 'Coast of Maine' borders. Which molecular mechanism is responsible for this condition?

Explanation

Correct Answer: B

The patient has McCune-Albright syndrome (Fibrous Dysplasia, café-au-lait spots, and endocrinopathies). This is caused by a somatic mutation in the GNAS1 gene, which leads to constitutive activation of the Gs-alpha protein, resulting in overproduction of intracellular cAMP and anarchic bone development.

Question 16

Histological examination of a lesion in a patient with Fibrous Dysplasia typically reveals which of the following features?

Explanation

Correct Answer: B

Fibrous dysplasia is characterized histologically by 'Chinese character' or 'alphabet soup' patterns of irregular woven bone trabeculae. A key diagnostic feature is the absence of osteoblastic rimming, which helps differentiate it from other fibro-osseous lesions like ossifying fibroma.

Question 17

A 14-year-old patient with polyostotic fibrous dysplasia presents with a progressive varus deformity of the proximal femur (Shepherd's crook deformity). What is the most appropriate surgical strategy to prevent recurrence?

Explanation

Correct Answer: C

For the Shepherd's crook deformity in Fibrous Dysplasia, the gold standard is a valgus-producing osteotomy to improve mechanical alignment, stabilized by an intramedullary device. Intramedullary nails are preferred over plates because they provide better support for the entire weakened diaphysis and are less likely to fail in the 'ground-glass' bone.

Question 18

Dysplasia Epiphysealis Hemimelica (Trevor disease) is best described as:

Explanation

Correct Answer: B

Trevor disease (DEH) is a rare developmental disorder characterized by asymmetric osteochondromatous overgrowth of one or more epiphyses. It most commonly affects the medial side of the ankle or knee joints.

Question 19

An adult patient with Multiple Hereditary Exostoses (MHE) undergoes an MRI for a rapidly enlarging pelvic mass. Which of the following MRI findings is most suggestive of malignant transformation to chondrosarcoma?

Explanation

Correct Answer: C

In a skeletally mature patient, a cartilage cap thickness greater than 1.5 to 2.0 cm on MRI is highly suspicious for malignant transformation into secondary chondrosarcoma. Other signs include new pain, growth after skeletal maturity, and associated soft tissue masses.

Question 20

Which of the following statements regarding the inheritance of Enchondromatosis (Ollier disease) is correct?

Explanation

Correct Answer: C

Ollier disease and Maffucci syndrome are not inherited. They are sporadic conditions caused by somatic mosaic mutations, most commonly in the IDH1 or IDH2 genes, occurring early in development.

Question 21

A 4-year-old child with achondroplasia presents with new-onset sleep apnea, lower extremity hyperreflexia, and clumsiness. Which of the following is the most appropriate next step in management?

Explanation

Central sleep apnea and hyperreflexia in achondroplasia strongly suggest cervicomedullary compression at the foramen magnum. MRI of the craniocervical junction is critical to evaluate the need for suboccipital decompression.

Question 22

A patient with Osteogenesis Imperfecta Type I typically exhibits blue sclerae, hearing loss, and recurrent fractures. Which of the following best describes the underlying collagen defect in this specific type?

Explanation

Osteogenesis Imperfecta Type I is a quantitative defect resulting in a decreased amount of structurally normal type I collagen. Types II, III, and IV typically involve qualitative defects (structurally abnormal collagen).

Question 23

A 7-year-old child presents with a waddling gait, bilateral knee pain, and restricted joint mobility. Radiographs demonstrate delayed, irregular ossification of multiple epiphyses and a characteristic "double-layered" patella. A mutation in which gene is most commonly associated with this phenotype?

Explanation

Multiple Epiphyseal Dysplasia (MED) is most commonly caused by mutations in the COMP (Cartilage Oligomeric Matrix Protein) gene. A double-layered patella on lateral knee radiographs is highly characteristic of MED.

Question 24

A 5-year-old with short-trunk dwarfism presents with neck pain. Radiographs reveal delayed ossification of the pubic symphysis, coxa vara, and an absent odontoid process. What is the most likely underlying genetic defect?

Explanation

Spondyloepiphyseal Dysplasia Congenita (SEDC) presents at birth with short-trunk dwarfism, coxa vara, and cervical instability due to odontoid hypoplasia. It is an autosomal dominant disorder caused by a mutation in the COL2A1 gene.

Question 25

A 14-year-old male presents with delayed eruption of secondary teeth and the ability to easily approximate his shoulders anteriorly. Radiographs show delayed ossification of the cranial sutures and midline structures. Which gene is mutated in this condition?

Explanation

Cleidocranial dysplasia is characterized by hypoplastic or absent clavicles, delayed suture closure, and dental anomalies. It is caused by an autosomal dominant mutation in the RUNX2 (CBFA1) transcription factor gene.

Question 26

A newborn presents with short limbs, severe rigid clubfeet, a "hitchhiker" thumb, and cystic swelling of the pinnae. Cervical kyphosis is noted on radiographs. What is the underlying defective mechanism?

Explanation

Diastrophic dysplasia is caused by a mutation in the SLC26A2 gene, resulting in defective intracellular sulfate transport. Clinical hallmarks include hitchhiker thumbs, severe clubfeet, and cauliflower ears.

Question 27

A 3-year-old child presents with disproportionate short-limb dwarfism and joint laxity. Unlike classic achondroplasia, the child's facial features and head circumference are completely normal. Radiographs show platyspondyly with anterior tongue-like projections. What best distinguishes this diagnosis from achondroplasia?

Explanation

Pseudoachondroplasia presents with normal craniofacial features and is typically not recognized until 2 to 3 years of age, unlike achondroplasia which is obvious at birth. It is caused by a COMP gene mutation.

Question 28

A child with short-trunk dwarfism exhibits a flat midface, prominent joints, and a cleft palate. Radiographs show broad, shortened long bones described as "dumbbell-shaped" femora, and coronal clefts in the vertebral bodies. What is the most likely diagnosis?

Explanation

Kniest dysplasia is a type II collagenopathy characterized by a flat midface, cleft palate, and prominent joints. The radiographic hallmarks include dumbbell-shaped femora and coronal clefts of the vertebrae.

Question 29

A 6-year-old with short stature, corneal clouding, and normal intelligence presents with progressive cervical myelopathy. Radiographs reveal generalized platyspondyly and severe odontoid hypoplasia. Urine analysis will most likely show elevated levels of which glycosaminoglycan?

Explanation

Morquio syndrome (Mucopolysaccharidosis Type IV) involves a deficiency of N-acetylgalactosamine-6-sulfatase (GALNS), leading to the accumulation of keratan sulfate. Odontoid hypoplasia causing cervical instability is a critical orthopedic manifestation.

Question 30

A 9-year-old girl presents with a pathologic proximal femur fracture, precocious puberty, and large, irregular "coast of Maine" café-au-lait spots. The underlying pathophysiology involves which of the following?

Explanation

McCune-Albright syndrome is characterized by polyostotic fibrous dysplasia, endocrine abnormalities, and café-au-lait spots. It is caused by a somatic activating mutation in the GNAS gene, leading to elevated cAMP levels.

Question 31

An infant with malignant osteopetrosis presents with cranial nerve palsies, hepatosplenomegaly, and recurrent fractures. Radiographs show a "bone-in-bone" appearance. What is the primary cellular defect responsible for this condition?

Explanation

Osteopetrosis is caused by defective osteoclast function, specifically the inability to acidify the resorption pit due to mutations affecting the ruffled border (e.g., Carbonic Anhydrase II or TCIRG1 mutations).

Question 32

A 25-year-old male presents with recurrent transverse femur fractures. He exhibits short stature, prominent eyes, delayed closure of cranial sutures, and acro-osteolysis of the distal phalanges. Deficiency of which enzyme is the root cause?

Explanation

Pycnodysostosis is an autosomal recessive lysosomal storage disease caused by Cathepsin K deficiency. Key features include osteosclerosis, acro-osteolysis, delayed suture closure, and brittle bones.

Question 33

A female infant is born with asymmetric limb shortening, ichthyosiform skin lesions, and early-onset cataracts. Radiographs demonstrate punctate calcifications in the epiphyses of the long bones. What is the most likely diagnosis?

Explanation

Chondrodysplasia punctata (specifically Conradi-Hünermann syndrome) is an X-linked dominant condition characterized by "stippled" epiphyses, asymmetric limb shortening, cataracts, and scaly skin lesions.

Question 34

A newborn presents with respiratory distress, severe anterolateral bowing of the femora and tibiae, and pretibial skin dimpling. Genetic testing reveals a mutation in SOX9. This condition is most closely associated with which of the following anomalies?

Explanation

Campomelic dysplasia is caused by a SOX9 mutation and is classically associated with severe bowing of the long bones, pretibial dimples, and sex reversal where 75% of XY individuals appear phenotypically female.

Question 35

A 4-year-old child presents with short stature, a waddling gait, and progressive coxa vara and bowlegs. Radiographs show widened, irregular metaphyseal growth plates at the knees, but the epiphyses and spine are completely normal. A mutation in which gene is responsible?

Explanation

Metaphyseal chondrodysplasia, Schmid type, is caused by a mutation in the COL10A1 gene (encoding Type X collagen), which is restricted to the hypertrophic zone of the growth plate. It presents with metaphyseal flaring and normal epiphyses/spine.

Question 36

An infant is born with a remarkably long, narrow trunk, exceptionally short extremities, and a prominent coccygeal tail-like appendage. Over the next few years, the child develops severe kyphoscoliosis, reversing the proportions to a short-trunk appearance. Which gene is most likely mutated?

Explanation

Metatropic dysplasia is caused by mutations in the TRPV4 gene. It is uniquely characterized by a change in body proportions over time: starting with a long trunk and short limbs, then developing severe kyphoscoliosis resulting in a short trunk.

Question 37

A 35-year-old male with achondroplasia presents with progressive neurogenic claudication and lower extremity weakness. What is the primary anatomic cause of spinal stenosis in this condition?

Explanation

Spinal stenosis in achondroplasia is primarily a bony developmental issue characterized by congenitally short pedicles and a progressive decrease in the interpedicular distance from the upper to lower lumbar spine.

Question 38

A child from an isolated Amish community presents with disproportionate short stature, postaxial polydactyly of the hands, and dysplastic nails. Which of the following cardiac defects is most classically associated with this specific skeletal dysplasia?

Explanation

Ellis-van Creveld syndrome (Chondroectodermal Dysplasia) features short limbs, postaxial polydactyly, and nail/teeth dysplasia. Cardiac anomalies occur in 50-60% of patients, classically a single atrium or ASD.

Question 39

In a pediatric patient with severe Osteogenesis Imperfecta receiving cyclical intravenous pamidronate therapy, what is the exact cellular mechanism by which this medication improves bone density?

Explanation

Nitrogen-containing bisphosphonates (like pamidronate) act by inhibiting farnesyl pyrophosphate synthase in the mevalonate pathway. This prevents prenylation of small GTPases, ultimately causing osteoclast apoptosis.

Question 40

A 10-year-old boy presents with mild disproportionate short stature. Unlike patients with achondroplasia, his facial features are normal, but radiographs still show narrowing of the lower lumbar interpedicular distances. Which of the following best describes the pathogenesis of his condition?

Explanation

Hypochondroplasia is a milder form of dwarfism than achondroplasia and typically presents with normal facial features. It is caused by different mutations in the FGFR3 gene, most commonly in the proximal tyrosine kinase domain, whereas classic achondroplasia mutations occur in the transmembrane domain.

Question 41

A 4-year-old child with rhizomelic short stature, frontal bossing, and a trident hand presents for evaluation. The underlying pathophysiology involves a mutation in the FGFR3 gene. How does this mutation affect the physeal growth plate?

Explanation

Achondroplasia is caused by an autosomal dominant, gain-of-function mutation in FGFR3. This overactivity continuously inhibits chondrocyte proliferation and hypertrophy in the growth plate, leading to severely restricted endochondral ossification.

Question 42

An infant with achondroplasia presents with newly identified central sleep apnea, hyperreflexia, and clonus. What is the most critical next step in management?

Explanation

Neurological signs in an infant with achondroplasia strongly suggest foramen magnum stenosis with cervicomedullary compression. An MRI of the craniocervical junction is critical to evaluate for urgent surgical decompression.

Question 43

A 6-year-old boy with a history of recurrent fractures, blue sclerae, and dentinogenesis imperfecta is treated with intravenous pamidronate. What is the primary cellular mechanism of action of this medication in this patient?

Explanation

Bisphosphonates like pamidronate inhibit osteoclast-mediated bone resorption by inducing osteoclast apoptosis. This increases cortical thickness and trabecular bone density, thereby reducing fracture risk in Osteogenesis Imperfecta.

Question 44

A newborn presents with micromelic short stature, bilateral severe rigid clubfeet, "hitchhiker" thumbs, and swollen cauliflower ears. What is the genetic inheritance pattern and defective gene responsible for this condition?

Explanation

Diastrophic dysplasia is an autosomal recessive disorder caused by mutations in the SLC26A2 (DTDST) gene, leading to defective sulfate transport and impaired cartilage sulfation. Clinical hallmarks include hitchhiker thumbs, severe clubfeet, and cauliflower ears.

Question 45

A 5-year-old child presents with a waddling gait, short stature, and joint laxity, but has completely normal facial features. Radiographs show delayed epiphyseal ossification and platyspondyly with anterior tongue-like projections. What is the most likely defective gene?

Explanation

Pseudoachondroplasia is caused by mutations in the Cartilage Oligomeric Matrix Protein (COMP) gene. Unlike achondroplasia, patients have a normal facies and normal birth length, presenting later in childhood with growth retardation and profound joint laxity.

Question 46

A 7-year-old child with a short trunk, coxa vara, and severe myopia requires surgical clearance for strabismus correction. Which preoperative imaging is most critical for the anesthesiologist?

Explanation

Spondyloepiphyseal dysplasia congenita (SEDC) involves defects in type II collagen (COL2A1) and is highly associated with odontoid hypoplasia. Flexion-extension cervical radiographs are critical to rule out atlantoaxial instability prior to intubation.

Question 47

A 10-year-old child presents with bilateral hip pain and a waddling gait. Radiographs reveal flattened, fragmented capital femoral epiphyses bilaterally, but normal spine morphology. Which of the following best differentiates this condition from bilateral Legg-Calve-Perthes disease?

Explanation

Multiple Epiphyseal Dysplasia (MED) mimics bilateral Legg-Calve-Perthes disease but is distinguished by symmetric bilateral involvement, an autosomal dominant inheritance, and normal spine morphology (which differentiates it from spondyloepiphyseal dysplasia).

Question 48

A 14-year-old patient presents with a broad skull, delayed tooth eruption, and the ability to appose both shoulders anteriorly across the chest. Which gene mutation is responsible for this condition?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) gene, an essential transcription factor for osteoblast differentiation. It classically leads to absent or hypoplastic clavicles and delayed cranial suture closure.

Question 49

A 9-year-old girl presents with precocious puberty, café-au-lait spots with irregular borders, and a "shepherd's crook" deformity of the proximal femur. Which of the following accurately describes the underlying molecular defect?

Explanation

McCune-Albright syndrome is characterized by polyostotic fibrous dysplasia, café-au-lait spots (Coast of Maine), and endocrinopathies like precocious puberty. It is caused by a somatic (post-zygotic) activating mutation in the GNAS1 gene.

Question 50

A 6-year-old child presents with short-trunk dwarfism, corneal clouding, and severe genu valgum. Urine tests are positive for keratan sulfate. What is the most life-threatening orthopedic complication associated with this condition?

Explanation

Morquio syndrome (Mucopolysaccharidosis Type IV) is characterized by the accumulation of keratan sulfate. Odontoid hypoplasia is a hallmark finding, leading to severe atlantoaxial instability and the risk of fatal cervical myelopathy.

Question 51

A newborn presents with disproportionate short stature, postaxial polydactyly, hypoplastic nails, and a congenital heart defect. Which cardiac anomaly is most frequently associated with this specific skeletal dysplasia?

Explanation

Ellis-van Creveld syndrome (chondroectodermal dysplasia) is an autosomal recessive condition commonly found in the Amish population. It is strongly associated with cardiac defects, most notably a common atrium or atrial septal defects.

Question 52

An infant presents with diffuse bone sclerosis on radiographs demonstrating a "bone-within-bone" appearance, hepatosplenomegaly, and cranial nerve palsies. Which of the following is the definitive curative treatment for the malignant infantile form of this disease?

Explanation

Malignant infantile osteopetrosis is caused by defective osteoclasts (often a TCIRG1 mutation). Because osteoclasts are derived from the hematopoietic macrophage lineage, hematopoietic stem cell transplantation is the only definitive curative treatment.

Question 53

A 30-year-old man presents with a femur fracture after a minor fall. He has short stature, prominent eyes, an obtuse mandibular angle, and acro-osteolysis of the distal phalanges. Radiographs show generalized osteosclerosis. What is the underlying enzyme deficiency?

Explanation

Pycnodysostosis is an autosomal recessive lysosomal storage disease caused by a deficiency in Cathepsin K. This enzyme is crucial for osteoclast-mediated degradation of type I collagen, resulting in osteosclerosis and characteristic acro-osteolysis.

Question 54

A neonate presents with severe respiratory distress, anterior bowing of the tibiae, skin dimples over the tibial convexities, and hypoplastic scapulae. Chromosomal analysis reveals a 46,XY karyotype, but the patient has female external genitalia. What is the mutated gene?

Explanation

Camptomelic dysplasia is caused by a mutation in the SOX9 gene. This leads to severe skeletal bowing, respiratory distress due to tracheobronchomalacia, and often sex reversal (XY patients with female phenotype) due to SOX9's role in testis determination.

Question 55

A 2-year-old child with multiple café-au-lait macules and axillary freckling presents with an anterolateral bow of the tibia. Which of the following statements regarding the orthopedic management of this tibial deformity is most accurate?

Explanation

Anterolateral bowing of the tibia is highly associated with congenital pseudarthrosis of the tibia in Neurofibromatosis Type 1. Bracing is rarely curative, and surgical treatment requires aggressive resection of the pseudarthrosis, bone grafting, and rigid mechanical fixation.

Question 56

An infant presents with severe rickets-like bone deformities, failure to thrive, and paradoxical hypercalcemia. Laboratory testing shows markedly reduced serum alkaline phosphatase and elevated urinary phosphoethanolamine. What is the underlying genetic defect?

Explanation

Hypophosphatasia is an inherited disorder characterized by defective bone mineralization. It is caused by a mutation in the ALPL gene, resulting in a deficiency of tissue-nonspecific alkaline phosphatase (TNSALP) and accumulation of inorganic pyrophosphate.

Question 57

A 4-year-old boy presents with short stature, waddling gait, and bilateral coxa vara. Radiographs show widened and irregular physes with metaphyseal flaring, predominantly in the lower extremities. Spine radiographs are completely normal. The mutation associated with this condition involves which collagen type?

Explanation

Schmid type metaphyseal chondrodysplasia is an autosomal dominant condition caused by a mutation in the COL10A1 gene, leading to defective type X collagen. Type X collagen is localized to the hypertrophic zone of the growth plate, and the disorder typically spares the spine.

Question 58

According to the Sillence classification for Osteogenesis Imperfecta (OI), which type is considered perinatally lethal due to profound skeletal fragility and respiratory failure?

Explanation

Type II Osteogenesis Imperfecta is the most severe form and is uniformly lethal in the perinatal period. Infants typically suffer from severe pulmonary hypoplasia, multiple in utero fractures, and an extremely soft, unossified skull.

Question 59

A young child presents with severe short limb dwarfism, prominent eyes, and a highly arched palate. Laboratory tests reveal asymptomatic hypercalcemia and hypophosphatemia, with low levels of intact PTH. What is the pathogenesis of this condition?

Explanation

Jansen metaphyseal chondrodysplasia is a rare autosomal dominant disorder caused by an activating mutation in the PTH1R gene (PTH/PTHrP receptor). This causes ligand-independent signaling, mimicking primary hyperparathyroidism with hypercalcemia despite suppressed PTH.

Question 60

A 12-year-old with achondroplasia is undergoing bilateral lower extremity lengthening. Which of the following anatomic principles or complication risks must the surgeon consider unique to or highly prominent in achondroplastic patients?

Explanation

Patients with achondroplasia inherently have marked ligamentous laxity and disproportionate soft tissue tightness during limb lengthening. This places them at a significantly high risk for joint subluxations (especially the knee and hip) and contractures during distraction osteogenesis.

Question 61

A 4-year-old boy with a known diagnosis of achondroplasia presents for evaluation. His parents are concerned about his short limbs. Genetic analysis confirms a gain-of-function mutation in the FGFR3 gene. What is the primary cellular mechanism by which this mutation leads to the characteristic skeletal phenotype?

Explanation

Achondroplasia is caused by an activating (gain-of-function) mutation in FGFR3, which paradoxically inhibits chondrocyte proliferation. This primarily affects the proliferative zone of the physis, leading to diminished endochondral ossification and rhizomelic short stature.

Question 62

A 6-year-old child with Osteogenesis Imperfecta (OI) Type III presents with progressive anterolateral bowing of the bilateral femora and a history of multiple fractures. Which of the following surgical interventions is most appropriate for long-term management of the femoral deformities?

Explanation

In growing children with severe OI, corrective osteotomies ('shish kebab' technique) combined with telescopic intramedullary rods (e.g., Fassier-Duval) are the gold standard. They stabilize the bone, correct deformity, and elongate with growth to prevent recurrent fractures beyond the tip of the rod.

Question 63

A newborn presents with severe short-limbed dwarfism, bilateral clubfeet, hitchhiker thumbs, and cystic swelling of the external ears (cauliflower ears). Which of the following genes is most likely mutated in this patient?

Explanation

The clinical presentation is classic for Diastrophic Dysplasia. It is caused by an autosomal recessive mutation in the SLC26A2 (DTDST) gene, which encodes a sulfate transporter, leading to defective sulfation of cartilage proteoglycans.

Question 64

A 10-year-old boy presents with delayed closure of cranial sutures, an unusually broad forehead, and the ability to appose his shoulders anteriorly to the midline. A defect in the RUNX2 (CBFA1) gene is identified. This mutation primarily disrupts which of the following processes?

Explanation

The patient has Cleidocranial Dysplasia, which is caused by mutations in the RUNX2 (CBFA1) transcription factor. This gene is critical for osteoblast differentiation, and its defect predominantly impairs intramembranous ossification, affecting the clavicles and skull.

Question 65

A 5-year-old girl is evaluated for short stature and a waddling gait. Clinical examination reveals normal facial features and intelligence. Radiographs demonstrate delayed epiphyseal ossification and irregular, fragmented epiphyses. Genetic testing is most likely to reveal a mutation in which of the following?

Explanation

Pseudoachondroplasia is characterized by short stature, delayed epiphyseal ossification, and joint laxity, but unlike achondroplasia, facial features and head circumference are normal. It is caused by mutations in the COMP gene.

Question 66

A 14-year-old male presents with bilateral knee and hip pain. Radiographs show flattened and irregular epiphyses. A diagnosis of Multiple Epiphyseal Dysplasia (MED) is suspected over Spondyloepiphyseal Dysplasia (SED). Which radiographic finding strongly supports MED over SED?

Explanation

Multiple Epiphyseal Dysplasia (MED) and Spondyloepiphyseal Dysplasia (SED) both present with irregular epiphyses. However, MED selectively affects the epiphyses of long bones while largely sparing the spine (normal spine appearance), whereas SED prominently involves the spine (e.g., platyspondyly).

Question 67

A 7-year-old boy with Spondyloepiphyseal Dysplasia Congenita (SEDC) is scheduled for elective bilateral hip osteotomies. Due to his underlying condition, which of the following pre-operative evaluations is most critical?

Explanation

SEDC is a Type II collagenopathy (COL2A1 mutation) frequently associated with odontoid hypoplasia, which can lead to life-threatening atlantoaxial instability. Flexion-extension C-spine radiographs are mandatory prior to any procedure requiring intubation or general anesthesia.

Question 68

A 6-year-old child with Morquio Syndrome (Mucopolysaccharidosis Type IV) presents with genu valgum and a barrel chest. A defect in which of the following enzymes is responsible for this condition?

Explanation

Morquio Syndrome (MPS IV) Type A is caused by a deficiency of N-acetylgalactosamine-6-sulfatase, leading to the accumulation of keratan sulfate. Unlike many other mucopolysaccharidoses, intelligence is typically normal.

Question 69

An infant presents with profound anemia, hepatosplenomegaly, and recurrent fractures. Radiographs reveal uniformly dense bones with an absent medullary canal and a 'bone-within-a-bone' appearance. The underlying pathogenesis of this condition most commonly involves a deficiency in which of the following?

Explanation

The patient has infantile malignant osteopetrosis, characterized by dense but brittle bones due to failed osteoclast resorption. The most common underlying defect impairs the acidification of the osteoclast resorption pit, frequently involving carbonic anhydrase II or the TCIRG1 gene.

Question 70

A 9-year-old girl is diagnosed with McCune-Albright syndrome after presenting with polyostotic fibrous dysplasia, a large café-au-lait macule with irregular borders, and early breast development. The GNAS1 gene mutation in this disorder results in the overactivity of which of the following signaling pathways?

Explanation

McCune-Albright syndrome is caused by a somatic activating mutation in the GNAS1 gene. This leads to constitutive activation of the Gs-alpha protein, resulting in unregulated overproduction of intracellular cAMP, which drives the endocrine and skeletal abnormalities.

Question 71

A 3-year-old child presents with bowed legs, a waddling gait, and short stature. Radiographs show coxa vara and flared, irregular metaphyses. The spine and facial features are completely normal. Genetic testing reveals a mutation in COL10A1. What is the diagnosis?

Explanation

Schmid Metaphyseal Chondrodysplasia is an autosomal dominant condition caused by mutations in the COL10A1 gene, which affects type X collagen in the hypertrophic zone of the physis. It presents with short limbs, coxa vara, and metaphyseal flaring with a normal spine.

Question 72

A newborn is evaluated for skeletal disproportion. Radiographs reveal unusually short, dumbbell-shaped femora, coronal clefts in the vertebral bodies, and platyspondyly. Clinically, the infant has a cleft palate and prominent joints. What is the most likely diagnosis?

Explanation

Kniest Dysplasia is a Type II collagenopathy characterized clinically by prominent joints, cleft palate, and hearing loss. Radiographically, 'dumbbell-shaped' femora and coronal vertebral clefts are classic hallmark findings.

Question 73

A neonate is born with severe anterolateral bowing of the lower extremities, clubfeet, and ambiguous genitalia. Chromosomal analysis reveals a 46,XY karyotype despite female external genitalia. The infant shortly develops severe respiratory distress due to tracheomalacia. This condition is associated with a mutation in which gene?

Explanation

Camptomelic Dysplasia is caused by mutations in the SOX9 gene. It is characterized by severe bowing of the tibiae and femora, fatal respiratory distress from tracheobronchomalacia, and XY sex reversal (phenotypic females with an XY karyotype).

Question 74

A 5-year-old child exhibits profound short stature, prominent eyes, and micrognathia. Laboratory tests reveal marked hypercalcemia and hypophosphatemia, with low-normal levels of parathyroid hormone (PTH). Radiographs demonstrate severe metaphyseal irregularity and osteopenia. A mutation in which of the following is most likely?

Explanation

Jansen Metaphyseal Chondrodysplasia is caused by a ligand-independent, activating mutation in the PTH1R gene. This leads to unremitting signaling that mimics primary hyperparathyroidism (hypercalcemia, hypophosphatemia) despite low actual PTH levels, alongside severe metaphyseal dysplasia.

Question 75

A 35-year-old male with achondroplasia presents with progressive neurogenic claudication and bilateral lower extremity weakness. What is the primary anatomical cause of spinal stenosis in patients with this condition?

Explanation

In achondroplasia, defective endochondral ossification affects the neural arch development, leading to congenitally short pedicles and a progressive decrease in interpedicular distance from the upper to the lower lumbar spine, directly causing severe spinal stenosis.

Question 76

A 12-year-old male with short stature presents with a clavicle fracture after a minor fall. Examination reveals open cranial sutures, a prominent forehead, and short, broad hands. Radiographs display generalized osteosclerosis and distinct acro-osteolysis of the distal phalanges. What is the most likely enzymatic defect?

Explanation

Pycnodysostosis is a lysosomal storage disease of the bone caused by a deficiency in Cathepsin K, which osteoclasts need to degrade type I collagen. It causes osteosclerosis, delayed suture closure, and pathognomonic acro-osteolysis of the distal phalanges.

Question 77

A 2-year-old child presents with coarse facial features, severe thoracolumbar kyphosis (gibbus deformity), corneal clouding, and hepatosplenomegaly. Urine analysis shows elevated levels of both dermatan sulfate and heparan sulfate. Which enzyme is deficient in this patient?

Explanation

Hurler Syndrome (Mucopolysaccharidosis Type I) is caused by a deficiency in alpha-L-iduronidase, leading to the accumulation of dermatan and heparan sulfate. Clinically, it is differentiated from Hunter syndrome (MPS II) by the presence of corneal clouding.

Question 78

A neonate is evaluated for disproportionate short stature, postaxial polydactyly of the hands, and dysplastic fingernails. An echocardiogram is ordered due to a high association of a specific congenital heart defect with this syndrome. Which cardiac defect is most classically associated with Ellis-van Creveld syndrome?

Explanation

Ellis-van Creveld syndrome (chondroectodermal dysplasia) typically presents with short stature, postaxial polydactyly, and ectodermal defects (nails/teeth). Nearly 50-60% of patients have congenital heart disease, most commonly an atrial septal defect or a single atrium.

Question 79

In severe, lethal Osteogenesis Imperfecta (Type II), the most common molecular defect involves which of the following alterations in collagen synthesis?

Explanation

Lethal OI (Type II) is usually caused by a qualitative defect where a single base substitution replaces glycine (the smallest amino acid) with a bulkier amino acid. This physically prevents the normal coiling of the collagen type I triple helix. Type I OI is typically a quantitative defect (decreased normal collagen).

Question 80

A newborn presents with short-limbed dwarfism, hitchhiker thumbs, cauliflower ears, and severe rigid clubfeet. What is the most critical orthopedic complication that must be monitored closely during early infancy in this condition?

Explanation

The patient has diastrophic dysplasia (SLC26A2 mutation). Cervical kyphosis is common and can progress to severe spinal cord compression or resolve spontaneously, making close monitoring critical.

Question 81

A 10-year-old child with normal intelligence presents with a prominent forehead, delayed eruption of secondary teeth, and the ability to appose both shoulders at the anterior midline. A mutation in which of the following genes is the underlying cause?

Explanation

This classic presentation describes Cleidocranial Dysplasia, characterized by hypoplastic or absent clavicles. It is caused by a mutation in the RUNX2 (also known as CBFA1) gene, essential for osteoblast differentiation.

Question 82

An 8-year-old child is evaluated for joint pain and a waddling gait. Radiographs demonstrate small, irregular epiphyses with a normal spine. A lateral knee radiograph reveals a pathognomonic 'double-layer' patella. A mutation in which gene is most likely responsible?

Explanation

The 'double-layer' patella is pathognomonic for Multiple Epiphyseal Dysplasia (MED). The most common mutation associated with the autosomal dominant form of MED is the COMP gene.

Question 83

A 45-year-old male with achondroplasia presents with progressively worsening neurogenic claudication. What is the primary anatomical etiology of spinal stenosis in this patient population?

Explanation

Spinal stenosis in achondroplasia is primarily driven by congenital narrowing of the spinal canal due to shortened pedicles and an abnormal decrease in interpedicular distance from L1 to L5.

Question 84

A 5-year-old child with short-trunk dwarfism, coxa vara, and myopia requires surgery for an umbilical hernia. Based on the likely diagnosis, what critical screening must be performed prior to general anesthesia?

Explanation

The presentation is classic for Spondyloepiphyseal Dysplasia Congenita (COL2A1 mutation). These patients have odontoid hypoplasia, leading to atlantoaxial instability, necessitating flexion-extension C-spine films before intubation.

Question 85

A 12-year-old female sustains a subtrochanteric femur fracture. She has a history of anemia, hepatosplenomegaly, and recurrent fractures. Radiographs reveal a uniform 'bone-within-bone' appearance. The underlying pathogenesis involves failure of which cellular mechanism?

Explanation

This patient has Osteopetrosis, characterized by dense but brittle bones. The disease is caused by defective osteoclast function (often TCIRG1 or CLCN7 mutations) leading to an absent or impaired ruffled border and failure of bone resorption.

Question 86

An 8-year-old girl with precocious puberty and large café-au-lait spots presents with a progressive 'shepherd's crook' deformity of the proximal femur. What is the most appropriate surgical management for this deformity?

Explanation

The diagnosis is McCune-Albright syndrome (Fibrous Dysplasia). Lesions resorb bone grafts and often cause plate/screw failure; the gold standard is valgus-producing osteotomy stabilized with an intramedullary nail.

Question 87

A 6-year-old boy presents with a short trunk, severe genu valgum, corneal clouding, and normal intelligence. Urine analysis shows elevated keratan sulfate. What specific cervical spine pathology is most characteristic of this condition?

Explanation

This describes Morquio Syndrome (Mucopolysaccharidosis Type IV). Odontoid hypoplasia is a hallmark of Morquio syndrome, creating a high risk of atlantoaxial instability and catastrophic spinal cord injury.

Question 88

A 4-year-old boy presents with short limbs and a waddling gait but normal facial features. Radiographs show delayed epiphyseal ossification, irregular flared metaphyses, and a normal skull. What is the inheritance pattern and associated gene for this condition?

Explanation

The patient has pseudoachondroplasia, distinguished from achondroplasia by normal facies. It is an autosomal dominant condition caused by mutations in the COMP (Cartilage Oligomeric Matrix Protein) gene.

Question 89

An 18-month-old infant is noted to have anterolateral bowing of the left tibia. She also has multiple café-au-lait spots and axillary freckling. What is the most appropriate initial orthopedic management to prevent progression to pseudoarthrosis?

Explanation

Anterolateral bowing of the tibia is associated with Neurofibromatosis Type 1 (NF1). The standard of care before a fracture occurs is full-time total contact bracing (clamshell brace) to protect the tibia from fracturing and progressing to recalcitrant pseudoarthrosis.

Question 90

A 15-year-old male with recurrent fractures presents with short stature, a prominent forehead, delayed cranial suture closure, and acro-osteolysis of the distal phalanges. Radiographs reveal generalized osteosclerosis. This condition is caused by a mutation affecting which of the following?

Explanation

This is pycnodysostosis, an autosomal recessive lysosomal storage disease caused by a mutation in Cathepsin K. Key features distinguishing it from osteopetrosis include delayed suture closure, normal medullary canals, and acro-osteolysis.

Question 91

A newborn is diagnosed with Ellis-van Creveld syndrome (Chondroectodermal Dysplasia). In addition to short limbs, polydactyly, and an atrial septal defect, which of the following lower extremity deformities is most likely to develop as the child grows?

Explanation

Ellis-van Creveld syndrome is characterized by a defect in the lateral aspect of the proximal tibial physis. This depression of the lateral tibial plateau predictably leads to severe progressive genu valgum.

Question 92

A neonatologist requests orthopedic consultation for a newborn diagnosed with Osteogenesis Imperfecta (OI) based on multiple intrauterine fractures and profound long bone deformities. According to the Sillence classification, which type of OI is uniformly fatal in the perinatal period?

Explanation

Sillence Type II Osteogenesis Imperfecta is the most severe form and is uniformly fatal in the perinatal period due to severe pulmonary hypoplasia and profound skeletal fragility.

Question 93

A 3-year-old child presents with a short trunk, flat midface, prominent joints, hearing loss, and myopia. Radiographs reveal characteristic 'dumbbell-shaped' femora and coronal clefts in the vertebral bodies. Which gene is most likely mutated in this patient?

Explanation

The clinical and radiographic presentation is classic for Kniest Dysplasia, which is a Type II collagenopathy caused by mutations in the COL2A1 gene. 'Dumbbell-shaped' femora are characteristic.

Question 94

An infant born with multiple large joint dislocations (bilateral knees, hips, and elbows), a depressed nasal bridge, and spatulate thumbs is diagnosed with Larsen syndrome. Which spinal abnormality must be urgently evaluated to prevent catastrophic myelopathy?

Explanation

Patients with Larsen syndrome (FLNB mutation) have a high incidence of severe, progressive cervical kyphosis. This can lead to fatal or severely morbid spinal cord compression if not identified and treated early.

Question 95

A 6-month-old infant with achondroplasia exhibits hypotonia, frequent apneic episodes, and hyperreflexia in the lower extremities. Which of the following is the most appropriate next step in management?

Explanation

Infants with achondroplasia are at high risk for foramen magnum stenosis, which causes cervicomedullary compression leading to sleep apnea, hypotonia, and hyperreflexia. Urgent MRI of the craniocervical junction is required.

Question 96

A 4-year-old presents with short stature, a waddling gait, bilateral coxa vara, and genu varum. Radiographs display widened and irregular metaphyses, but perfectly normal epiphyses and spine. Genetic testing reveals a mutation in COL10A1. What is the diagnosis?

Explanation

Schmid Metaphyseal Chondrodysplasia is caused by a mutation in COL10A1 (Type X collagen). It presents with coxa vara and genu varum, affecting only the metaphyses while sparing the epiphyses and spine.

Question 97

A 5-year-old boy presents with a hard, painless swelling on the medial aspect of his right ankle, limiting range of motion. Radiographs show an irregular, lobulated bony mass arising exclusively from the medial epiphysis of the distal tibia. What is the most likely diagnosis?

Explanation

Trevor disease (Dysplasia Epiphysealis Hemimelica) is characterized by an asymmetric osteochondroma-like overgrowth arising from an epiphysis, most commonly affecting the ankle or knee in young children.

Question 98

A 4-year-old with Osteogenesis Imperfecta Type III is undergoing treatment with intravenous pamidronate. What is the primary cellular mechanism of action of this pharmacological therapy?

Explanation

Bisphosphonates like pamidronate are analogs of pyrophosphate that bind to hydroxyapatite and are ingested by osteoclasts. They induce osteoclast apoptosis, thereby inhibiting bone resorption and increasing overall bone density.

Question 99

A phenotypic female newborn presents with severe anterior bowing of the tibiae, pretibial skin dimples, and respiratory distress due to tracheomalacia. Karyotype analysis surprisingly reveals a 46,XY chromosomal pattern. A mutation in which gene is responsible for this condition?

Explanation

This presentation describes Campomelic Dysplasia, which features severe bowing of long bones, pretibial dimples, and tracheomalacia. It is caused by a mutation in the SOX9 gene, which frequently leads to sex reversal in 46,XY individuals.

Arab Board Orthopedic B Review | Dr Hutaif General Orth -...

Arab Board Orthopedic Exam: Skeletal Dysplasias MCQs

Arab Board Orthopedic Exam: Skeletal Dysplasias MCQs

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