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1. General Principles & Basic Science MCQs

Practice Set 487 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 9721

Topic: 1. General Principles & Basic Science

Review the provided midfoot osteotomy illustration.

When utilizing a V-osteotomy for the correction of a multiplanar midfoot deformity, positioning the apex of the 'V' exactly at the CORA ensures which of the following mechanical advantages?

. It allows for isolated translation without angular correction.
. It permits pure angular correction by acting as a native hinge, preventing unwanted dorsal or plantar translation of the bony margins.
. It automatically corrects simultaneous limb length discrepancies.
. It mandates a closing wedge mechanism exclusively.
. It bypasses Paley's Rule 1 and operates strictly under Rule 3.

Correct Answer & Explanation

. It permits pure angular correction by acting as a native hinge, preventing unwanted dorsal or plantar translation of the bony margins.

Explanation

Positioning the apex of a V-osteotomy at the CORA follows Paley's Rule 1. It acts as the center of rotation, allowing angular correction without translation, maximizing bone contact and stability.

Question 9722

Topic: 1. General Principles & Basic Science

A 16-year-old presents with an oblique plane deformity of the distal tibia, consisting of 20 degrees of varus and 15 degrees of recurvatum. To correct both deformities simultaneously with a single osteotomy and a circular frame, how should the hinge axis be oriented?

. Parallel to the mechanical axis of the tibia
. Perpendicular to the sagittal plane exclusively
. Perpendicular to the true plane of the maximum deformity
. Parallel to the true plane of the maximum deformity
. At a 45-degree angle to the transverse bisector line

Correct Answer & Explanation

. Perpendicular to the true plane of the maximum deformity

Explanation

An oblique plane deformity combines two orthogonal angulations into a single true plane of maximal deformity. Correcting it simultaneously requires placing the hinge axis exactly perpendicular to this single oblique plane.

Question 9723

Topic: Surgical Anatomy & Approaches

A patient presents with a severe post-traumatic valgus ankle with a depressed lateral tibial plafond. Mechanical axis planning demonstrates the CORA is located directly at the articular surface. Which surgical approach is necessitated by this specific CORA location to normalize the mechanical axis and restore joint congruity?

. A standard supramalleolar closing wedge osteotomy (SMO)
. A supramalleolar opening wedge osteotomy with translation
. An intra-articular osteotomy elevating the depressed segment
. A distal fibular lengthening without tibial intervention
. A focal dome osteotomy 3 cm proximal to the joint line

Correct Answer & Explanation

. An intra-articular osteotomy elevating the depressed segment

Explanation

When the CORA is located at the joint line due to a depressed articular segment, extra-articular osteotomies cannot restore joint congruity. An intra-articular osteotomy is required to elevate the fragment and simultaneously correct the mechanical axis.

Question 9724

Topic: 1. General Principles & Basic Science

According to Paley's Rule 3, if an osteotomy is performed at the CORA, but the hinge is placed entirely off the transverse bisector line, what is the geometric consequence on the bone segments?

. They undergo pure angular correction and become collinear.
. They become collinear but with an unexpected length increase.
. They correct angularly but the axes become parallel and translated, causing a new mechanical axis deviation.
. They undergo pure translation without any change in angulation.
. They rotate axially without changing the mechanical axis deviation.

Correct Answer & Explanation

. They correct angularly but the axes become parallel and translated, causing a new mechanical axis deviation.

Explanation

Rule 3 dictates that a hinge placed off the transverse bisector line results in the anatomical axes becoming parallel but not collinear. This introduces a new translational deformity and alters the final mechanical axis.

Question 9725

Topic: 1. General Principles & Basic Science

A focal dome osteotomy is planned for a distal tibial deformity. To ensure the osteotomy allows angular correction without inducing any translation of the mechanical axis, what relationship must exist between the dome cut and the CORA?

. The radius of the dome cut must equal the distance to the knee joint.
. The center of the arc of the dome cut must perfectly coincide with the CORA.
. The osteotomy must be performed strictly distal to the CORA.
. The osteotomy must be performed strictly proximal to the CORA.
. The dome cut must be made completely orthogonal to the anatomical axis.

Correct Answer & Explanation

. The center of the arc of the dome cut must perfectly coincide with the CORA.

Explanation

A dome osteotomy rotates around its geometric center. To obey Paley's Rule 1 (or Rule 2 with collinear axes), the center of rotation of the dome cut must exactly match the CORA.

Question 9726

Topic: Biology, Genetics & Bone Healing

A 35-year-old man presents with chronic, dull, aching pain and stiffness in his right lower extremity. 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 genetic mutation associated with this condition?

. COMP
. GNAS
. FGFR3
. LEMD3
. SOX9

Correct Answer & Explanation

. LEMD3

Explanation

Correct Answer: LEMD3The clinical and radiographic description (dripping candle wax appearance along the cortex) is pathognomonic for Melorheostosis. This is a rare, non-hereditary sclerosing bone dysplasia characterized by anarchic, hyperostotic bone formation. It is frequently associated with somatic loss-of-function mutations in the LEMD3 gene (also known as MAN1), which is involved in bone morphogenetic protein (BMP) and TGF-beta signaling. LEMD3 mutations are also associated with Osteopoikilosis and Buschke-Ollendorff syndrome.

Question 9727

Topic: Biology, Genetics & Bone Healing

A 35-year-old patient presents with chronic, deep aching pain in the lower extremity and joint contractures. Radiographs demonstrate dense, irregular, eccentric hyperostosis along the cortex of the femur and tibia, resembling dripping candle wax. Mutations in which of the following genes are most commonly associated with this condition?

. GNAS1
. EXT2
. LEMD3
. SOX9
. RUNX2

Correct Answer & Explanation

. LEMD3

Explanation

Correct Answer: LEMD3The clinical and radiographic description is classic for melorheostosis, a rare sclerosing bone dysplasia characterized by anarchic hyperostosis that resembles 'dripping candle wax' or 'flowing wax' on radiographs. It typically affects one limb (monomelic) and can cause severe pain and joint contractures. Melorheostosis, along with osteopoikilosis and Buschke-Ollendorff syndrome, has been linked to loss-of-function mutations in the LEMD3 gene (also known as MAN1), which encodes an inner nuclear membrane protein that interacts with BMP and TGF-beta signaling pathways. GNAS1 is linked to fibrous dysplasia, EXT2 to multiple hereditary exostoses, SOX9 to campomelic dysplasia, and RUNX2 to cleidocranial dysplasia.

Question 9728

Topic: Biology, Genetics & Bone Healing

A 28-year-old female with polyostotic fibrous dysplasia complains of severe, persistent bone pain in her lower extremities that is refractory to NSAIDs. Radiographs show stable, ground-glass expansile lesions without impending fracture. Which of the following pharmacological interventions is most appropriate as the next step in management?

. Oral corticosteroids
. Subcutaneous teriparatide
. Intravenous bisphosphonates
. Intravenous antibiotics
. Oral methotrexate

Correct Answer & Explanation

. Intravenous bisphosphonates

Explanation

Correct Answer: Intravenous bisphosphonatesIn patients with fibrous dysplasia, bone pain is a common and debilitating symptom. When conservative measures like NSAIDs fail, intravenous bisphosphonates (such as pamidronate or zoledronic acid) are the pharmacological treatment of choice. Bisphosphonates inhibit osteoclastic bone resorption, which is upregulated in fibrous dysplasia lesions, thereby reducing bone turnover and significantly alleviating bone pain. They may also improve the radiographic appearance of the lesions in some cases. Teriparatide (an anabolic agent) is contraindicated as it stimulates bone turnover. Corticosteroids and methotrexate have no role in the management of fibrous dysplasia.

Question 9729

Topic: Biology, Genetics & Bone Healing

A biopsy is taken from a lytic, expansile lesion in the proximal femur of a 20-year-old patient. Histological examination reveals irregular, woven bone trabeculae lacking osteoblastic rimming, set within a bland fibrous stroma. The trabeculae are often described as having a 'Chinese character' appearance. Which of the following molecular abnormalities is the primary driver of this pathology?

. Defective synthesis of heparan sulfate
. Constitutive activation of the adenylate cyclase pathway
. Loss of function of the retinoblastoma (Rb) tumor suppressor protein
. Overexpression of fibroblast growth factor receptor 3 (FGFR3)
. Defective osteoclast-mediated bone resorption

Correct Answer & Explanation

. Constitutive activation of the adenylate cyclase pathway

Explanation

Correct Answer: Constitutive activation of the adenylate cyclase pathwayThe histology describes Fibrous Dysplasia (woven bone lacking osteoblastic rimming in a fibrous stroma, 'Chinese characters'). Fibrous dysplasia is caused by a somatic, post-zygotic activating mutation in the GNAS gene. This mutation affects the alpha subunit of the stimulatory G protein (Gs-alpha), leading to constitutive activation of adenylate cyclase and an overproduction of intracellular cyclic AMP (cAMP). This downstream signaling cascade results in the anarchic proliferation of undifferentiated mesenchymal cells that fail to mature into normal osteoblasts. Defective heparan sulfate synthesis is seen in Multiple Hereditary Exostoses.

Question 9730

Topic: 1. General Principles & Basic Science

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

. Osteopathia striata
. Melorheostosis
. Osteopoikilosis
. Diaphyseal dysplasia (Engelmann disease)
. Osteopetrosis

Correct Answer & Explanation

. Melorheostosis

Explanation

Correct Answer: MelorheostosisMelorheostosis is a rare, non-hereditary sclerosing bone dysplasia characterized by linear, eccentric hyperostosis of the cortex that resembles 'dripping candle wax' on radiographs. It typically affects a single limb (monomelic) and can cause severe pain, joint contractures, and soft tissue ossification. It is associated with somatic mutations in the LEMD3 gene or the MAP2K1 gene. Osteopoikilosis presents as multiple small sclerotic bone islands. Osteopathia striata presents as linear striations parallel to the long axis of the bone.

Question 9731

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy with Ollier disease has a 6 cm leg length discrepancy due to extensive involvement of his left femur. He undergoes a femoral lengthening procedure using an external fixator. Which of the following complications is most specifically associated with distraction osteogenesis in patients with this condition?

. Formation of cartilaginous tissue within the distraction gap leading to delayed or non-union.
. Premature consolidation of the regenerate bone.
. Uncontrollable pin tract infections due to underlying immunodeficiency.
. Rapid malignant transformation of the enchondromas during the distraction phase.
. Severe heterotopic ossification around the pin sites.

Correct Answer & Explanation

. Formation of cartilaginous tissue within the distraction gap leading to delayed or non-union.

Explanation

Correct Answer: Formation of cartilaginous tissue within the distraction gap leading to delayed or non-union.Ollier disease (multiple enchondromatosis) involves a defect in normal endochondral ossification, where chondrocytes fail to undergo normal hypertrophy and apoptosis, leaving islands of unmineralized cartilage in the metaphyses and diaphyses. When distraction osteogenesis is performed through or near these dysplastic segments, the regenerate tissue often forms abnormal cartilaginous tissue rather than normal woven bone. This leads to poor quality regenerate, delayed consolidation, and a high risk of non-union or fracture of the regenerate after frame removal. Slower distraction rates and prolonged frame time are often required.

Question 9732

Topic: Biology, Genetics & Bone Healing
A 2-year-old child with multiple café-au-lait spots presents with anterolateral bowing of the tibia. Radiographs demonstrate medullary sclerosis and a frank pseudarthrosis at the junction of the middle and distal thirds of the tibia (Crawford Type IV). What is the most critical biological factor contributing to the failure of bone healing in this specific condition?
. Hyperactive osteoclast function due to RANKL overexpression
. Thickened, highly cellular, and fibromatosis-like periosteum
. Complete absence of the nutrient artery to the tibia
. Defective type I collagen synthesis
. Overexpression of FGFR3 inhibiting chondrocyte proliferation

Correct Answer & Explanation

. Thickened, highly cellular, and fibromatosis-like periosteum

Explanation

Congenital pseudarthrosis of the tibia (CPT) is strongly associated with Neurofibromatosis type 1 (NF1). The primary biological barrier to healing in CPT is the presence of a thickened, hamartomatous, fibromatosis-like periosteum surrounding the pseudarthrosis site. This abnormal periosteum restricts blood supply, causes local bone resorption, and mechanically prevents callus formation. Successful surgical treatment requires complete excision of this diseased periosteum (hamartoma) down to healthy tissue.

Question 9733

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with painless anterior bowing of the tibia. Radiographs show an intracortical, multi-loculated radiolucent lesion in the anterior diaphyseal cortex of the tibia with sclerotic margins. Biopsy shows fibrous tissue with trabeculae of woven bone rimmed by active osteoblasts. Which of the following is the most appropriate initial management?

. Immediate wide en bloc resection
. Curettage and bone grafting
. Observation and bracing
. Radiation therapy
. Intramedullary nailing

Correct Answer & Explanation

. Observation and bracing

Explanation

Correct Answer: Observation and bracingThe clinical and radiographic picture describes osteofibrous dysplasia (Campanacci disease), which almost exclusively affects the anterior cortex of the tibia in children under 10 years old. Histologically, it is distinguished from fibrous dysplasia by the presence of osteoblastic rimming around the bone trabeculae. The natural history of osteofibrous dysplasia is stabilization or regression after skeletal maturity. Early surgical intervention (curettage or excision) carries an extremely high recurrence rate. Therefore, observation and bracing (to prevent fracture) is the recommended initial management, delaying surgery until skeletal maturity unless there is an impending fracture or severe progressive deformity.

Question 9734

Topic: Biology, Genetics & Bone Healing

A 30-year-old male with polyostotic fibrous dysplasia complains of severe, persistent bone pain in his lower extremities that is refractory to NSAIDs. Radiographs show stable ground-glass lesions without impending fractures or significant deformity. Which of the following pharmacological interventions is most appropriate to manage his bone pain?

. Intravenous bisphosphonates
. Oral corticosteroids
. Methotrexate
. Teriparatide (PTH analog)
. Denosumab

Correct Answer & Explanation

. Intravenous bisphosphonates

Explanation

Correct Answer: Intravenous bisphosphonatesIn fibrous dysplasia, the anarchic fibrous tissue produces factors that stimulate osteoclastic bone resorption, leading to bone pain and structural weakness. Intravenous bisphosphonates (such as pamidronate or zoledronic acid) are the mainstay of medical therapy for symptomatic fibrous dysplasia. They inhibit osteoclast activity, significantly reducing bone pain and potentially improving the radiographic appearance of the lesions. Teriparatide is contraindicated as it stimulates bone turnover, and denosumab is generally reserved for giant cell tumors or refractory cases under strict protocols due to rebound hypercalcemia risks.

Question 9735

Topic: Biology, Genetics & Bone Healing

A neonate is evaluated for short limbs, bilateral severe rigid clubfeet, hitchhiker thumbs, and cystic swelling of the ear pinnae. Genetic testing reveals a mutation in the SLC26A2 gene. This defect primarily impairs which of the following cellular processes?

. Type II collagen synthesis
. Cartilage oligomeric matrix protein secretion
. Fibroblast growth factor receptor signaling
. Sulfate transport into chondrocytes
. Transcription of Runx2

Correct Answer & Explanation

. Sulfate transport into chondrocytes

Explanation

Diastrophic dysplasia is caused by a mutation in SLC26A2 (DTDST), leading to defective intracellular sulfate transport and underminosulfation of cartilage proteoglycans. Clinical hallmarks include hitchhiker thumbs, cauliflower ears, and severe clubfeet.

Question 9736

Topic: Biology, Genetics & Bone Healing

A 5-year-old presents with frequent fractures, hepatosplenomegaly, and cranial nerve palsies. Radiographs show diffusely dense bones with loss of medullary canals and an "Erlenmeyer flask" deformity of the distal femurs. The defective cellular mechanism primarily involves which of the following?

. Osteoblast terminal differentiation
. Chondrocyte proliferation in the resting zone
. Carbonic anhydrase II deficiency inhibiting lacunar acidification
. Cleavage of type I procollagen C-propeptides
. Type X collagen synthesis in the hypertrophic zone

Correct Answer & Explanation

. Carbonic anhydrase II deficiency inhibiting lacunar acidification

Explanation

Osteopetrosis is caused by defective osteoclast function, frequently due to mutations affecting carbonic anhydrase II or TCIRG1, preventing the acidification of the resorption pit. This leads to dense, brittle bones, marrow obliteration, and extramedullary hematopoiesis.

Question 9737

Topic: 1. General Principles & Basic Science

A 7-year-old boy presents with severe short stature, severe genu valgum, and a barrel chest. His intelligence is completely normal. Urine analysis reveals abnormally elevated levels of keratan sulfate. Which enzyme is deficient in this patient?

. Alpha-L-iduronidase
. Iduronate sulfatase
. N-acetylgalactosamine-6-sulfatase
. Glucocerebrosidase
. Sphingomyelinase

Correct Answer & Explanation

. N-acetylgalactosamine-6-sulfatase

Explanation

Morquio syndrome (Mucopolysaccharidosis Type IV) is caused by a deficiency of N-acetylgalactosamine-6-sulfatase (GALNS), leading to tissue accumulation of keratan sulfate. It is uniquely characterized by severe skeletal dysplasia, ligamentous laxity, and normal intelligence.

Question 9738

Topic: Biology, Genetics & Bone Healing

A patient presents with delayed closure of cranial sutures, supernumerary teeth, and the ability to appose their shoulders in the midline. Radiographs reveal absent clavicles and delayed ossification of the pubic symphysis. This condition is caused by a defect in a transcription factor essential for which of the following processes?

. Osteoclast bone resorption
. Chondrocyte hypertrophy in endochondral ossification
. Osteoblast differentiation and intramembranous ossification
. Type I collagen post-translational cross-linking
. Sulfate transport in the cartilage matrix

Correct Answer & Explanation

. Osteoblast differentiation and intramembranous ossification

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (CBFA1), a master transcription factor critical for osteoblast differentiation and intramembranous ossification. The defective ossification leads to absent clavicles, wide cranial sutures, and dental anomalies.

Question 9739

Topic: Biology, Genetics & Bone Healing

An infant presents with severe hypercalcemia, hypophosphatemia, and markedly irregular, cupped metaphyses on radiographs. Serum parathyroid hormone (PTH) and PTH-related peptide (PTHrP) levels are undetectable. What is the underlying pathophysiology?

. Vitamin D receptor mutation
. Ligand-independent activation of the PTH/PTHrP receptor
. Inactivating mutation of the calcium-sensing receptor
. Defect in renal 1-alpha-hydroxylase
. Hyperplasia of the parathyroid glands

Correct Answer & Explanation

. Ligand-independent activation of the PTH/PTHrP receptor

Explanation

Jansen metaphyseal chondrodysplasia is caused by an activating mutation in the PTH1R gene, leading to ligand-independent activation of the PTH/PTHrP receptor. This mimics hyperparathyroidism biologically, causing severe hypercalcemia and extensive metaphyseal abnormalities.

Question 9740

Topic: Biology, Genetics & Bone Healing

An infant is diagnosed with malignant infantile osteopetrosis. Radiographs reveal diffuse, uniform bone sclerosis ('bone within bone' appearance). Genetic testing reveals a mutation in the TCIRG1 gene. At the cellular level, what is the primary consequence of this mutation?

. Excessive proliferation of osteoblasts
. Failure of osteoclasts to undergo apoptosis
. Failure of osteoclasts to form a functioning ruffled border
. Defective cleavage of type I collagen propeptides
. Inability of osteocytes to signal mechanotransduction

Correct Answer & Explanation

. Failure of osteoclasts to form a functioning ruffled border

Explanation

TCIRG1 mutations cause a defect in the vacuolar proton pump required for acidification of the resorption pit. This prevents osteoclasts from forming a functioning ruffled border, halting normal bone resorption and leading to osteopetrosis.

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