Arab Board Orthopedic MCQs: Dysplasias, Tumors & Pediatrics

Question 1

A 4-year-old boy presents with short stature, rhizomelic shortening of the limbs, and frontal bossing. Radiographs show narrowing of the interpedicular distances in the lumbar spine. Based on the classification of skeletal dysplasias with predominantly metaphyseal involvement, which of the following genetic mutations is most likely responsible for this condition?

Explanation

Correct Answer: FGFR3

Achondroplasia is the most common form of short-limb dwarfism and is classified as a skeletal dysplasia with predominantly metaphyseal involvement. It is caused by an autosomal dominant, activating mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, which inhibits endochondral ossification.

Question 2

A 6-year-old girl is brought to the clinic with a history of multiple low-energy fractures. Examination reveals blue sclerae and dentinogenesis imperfecta. Which of the following best describes the underlying pathophysiology of her condition?

Explanation

Correct Answer: Defective synthesis of type I collagen

Osteogenesis Imperfecta is a skeletal dysplasia characterized by abnormalities of bone density and modeling defects. It is caused by quantitative or qualitative defects in type I collagen, typically due to mutations in the COL1A1 or COL1A2 genes, leading to bone fragility, blue sclerae, and hearing loss.

Question 3

A 7-year-old boy presents with a 3-day history of fever, malaise, and severe pain in his right distal femur. He refuses to bear weight. Laboratory tests show elevated CRP and ESR. In acute hematogenous pyogenic osteomyelitis in children, what is the most common anatomic location for the initial nidus of infection?

Explanation

Correct Answer: Metaphysis

Acute hematogenous pyogenic osteomyelitis in children most commonly begins in the metaphysis of long bones. This is due to the sluggish blood flow in the venous sinusoids of the metaphyseal region, which provides an ideal environment for bacterial seeding and proliferation.

Question 4

A 19-year-old male complains of a dull, aching pain in his left thigh that has been worsening over the past 6 months. The pain is characteristically worse at night and is dramatically relieved by ibuprofen. Radiographs reveal a small radiolucent nidus surrounded by dense sclerotic bone in the proximal femoral diaphysis. What is the most appropriate initial management?

Explanation

Correct Answer: Radiofrequency ablation or trial of NSAIDs

Osteoid osteoma is a benign primary bone tumor that classically presents with night pain relieved by NSAIDs due to high levels of prostaglandins within the nidus. Initial management can be medical (NSAIDs) or minimally invasive (radiofrequency ablation), which has largely replaced open surgical resection.

Question 5

A 14-year-old boy presents with a painful, swollen mass over his mid-tibial shaft. Radiographs show a permeative, destructive diaphyseal lesion with an 'onion-skin' periosteal reaction. Biopsy reveals small round blue cells. Which chromosomal translocation is most characteristic of this tumor?

Explanation

Correct Answer: t(11;22)

Ewing's sarcoma is a highly malignant primary bone tumor of childhood and adolescence. It is characterized by the t(11;22)(q24;q12) chromosomal translocation in approximately 85% of cases, resulting in the EWS-FLI1 fusion protein.

Question 6

A 6-year-old boy presents with a painless limp of 3 months duration. He has limited internal rotation and abduction of the right hip. Radiographs show sclerosis and fragmentation of the right capital femoral epiphysis. What is the primary etiology of this condition?

Explanation

Correct Answer: Idiopathic avascular necrosis of the capital femoral epiphysis

Legg-Calvé-Perthes disease is an idiopathic avascular necrosis of the proximal femoral epiphysis in children. It typically presents between ages 4 and 8 with a painless limp and restricted hip motion, particularly in internal rotation and abduction.

Question 7

A 13-year-old obese boy presents with left knee pain and a limp. Examination of the left hip reveals obligatory external rotation when the hip is passively flexed. Radiographs of the pelvis show a 'slip' of the proximal femoral epiphysis. What is the most appropriate definitive treatment?

Explanation

Correct Answer: In situ single screw fixation

Slipped Capital Femoral Epiphysis (SCFE) is best treated with in situ fixation to prevent further slippage and promote physeal closure. This is typically achieved using a single cannulated screw placed in the center of the epiphysis. Closed reduction is contraindicated as it increases the risk of avascular necrosis.

Question 8

A 45-year-old female typist presents with numbness and tingling in her thumb, index, and middle fingers, which frequently awakens her at night. Symptoms are reproduced by tapping over the volar aspect of the wrist. Which nerve is compressed in this condition?

Explanation

Correct Answer: Median nerve

Carpal Tunnel Syndrome is the most common entrapment neuropathy, involving compression of the median nerve as it passes through the carpal tunnel of the wrist. It classically presents with nocturnal paresthesias in the median nerve distribution and a positive Tinel's sign at the wrist.

Question 9

A 2-month-old female infant is evaluated in the clinic. The examiner places the infant supine, flexes the hips to 90 degrees, and gently abducts the hips while applying anterior pressure to the greater trochanters. A palpable 'clunk' is felt as the femoral head reduces into the acetabulum. What is the name of this clinical test?

Explanation

Correct Answer: Ortolani maneuver

The Ortolani maneuver is a clinical test used to screen for Developmental Dysplasia of the Hip (DDH). It reduces a dislocated hip back into the acetabulum, producing a palpable 'clunk'. The Barlow maneuver, conversely, attempts to dislocate a reducible hip.

Question 10

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femoral epiphysis extending to the subchondral bone, with a 'soap bubble' appearance. Biopsy shows multinucleated giant cells in a background of mononuclear stromal cells. What is the most likely diagnosis?

Explanation

Correct Answer: Giant cell tumor of bone

Giant cell tumor of bone typically occurs in young adults after physeal closure. It is classically located in the epiphysis or metaphysis of long bones, most commonly around the knee (distal femur, proximal tibia). Radiographically, it appears as an eccentric, lytic lesion extending to the subchondral bone.

Question 11

A 4-year-old boy presents with short stature, frontal bossing, and rhizomelic shortening of the limbs. Radiographs reveal narrowing of the interpedicular distances in the lumbar spine. Which of the following genetic mutations is most likely responsible for this condition?

Explanation

Correct Answer: FGFR3

The clinical and radiographic presentation is classic for achondroplasia, the most common form of short-limb dwarfism. It is caused by an activating mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, which inhibits chondrocyte proliferation in the proliferative zone of the physis. COL1A1 mutations cause Osteogenesis Imperfecta. COMP mutations are associated with Multiple Epiphyseal Dysplasia and Pseudoachondroplasia. RUNX2 mutations cause Cleidocranial Dysplasia, and EXT1 mutations are seen in Multiple Hereditary Exostoses.

Question 12

A 12-year-old girl is evaluated for delayed dental eruption and the ability to bring her shoulders together anteriorly. Radiographs show absent clavicles and delayed ossification of the pubic symphysis. What is the inheritance pattern and associated gene mutation for this syndrome?

Explanation

Correct Answer: Autosomal dominant, RUNX2 (CBFA1)

The patient has Cleidocranial Dysplasia, characterized by hypoplastic or absent clavicles, delayed closure of cranial sutures, delayed dental eruption, and widened pubic symphysis. It is an autosomal dominant condition caused by a mutation in the RUNX2 (also known as CBFA1) gene. This gene is a critical transcription factor for osteoblast differentiation and bone formation.

Question 13

A 16-year-old male complains of severe right thigh pain that is worse at night and dramatically relieved by ibuprofen. Radiographs show a 7 mm radiolucent nidus surrounded by dense reactive sclerosis in the proximal femoral diaphysis. What is the most appropriate initial definitive management if medical therapy fails or is not tolerated?

Explanation

Correct Answer: Radiofrequency ablation

The presentation is classic for an osteoid osteoma. The lesion produces high levels of prostaglandins, which explains the dramatic relief with NSAIDs. If medical management fails, is not tolerated, or the patient prefers definitive treatment, percutaneous radiofrequency ablation (RFA) is the treatment of choice. It offers high success rates with significantly less morbidity compared to open surgical resection (en bloc or curettage).

Question 14

An obese 13-year-old boy presents with a limp and vague knee pain. Examination reveals obligatory external rotation of the hip during passive flexion. Radiographs confirm a slipped capital femoral epiphysis (SCFE). In which direction does the femoral neck typically displace relative to the capital epiphysis in this condition?

Explanation

Correct Answer: Anterior and lateral

In a slipped capital femoral epiphysis (SCFE), the capital femoral epiphysis remains seated in the acetabulum while the femoral neck (metaphysis) displaces anteriorly and laterally. Although it is often conceptually described as the epiphysis slipping posteriorly and medially, the actual mechanical displacement involves the metaphysis moving anteriorly and laterally relative to the fixed epiphysis.

Question 15

A 32-year-old female presents with knee pain. Radiographs show an eccentric, lytic epiphyseal-metaphyseal lesion in the distal femur extending to the subchondral bone. Biopsy reveals multinucleated giant cells in a background of mononuclear stromal cells. Which of the following targeted therapies is most appropriate for unresectable or recurrent cases of this tumor?

Explanation

Correct Answer: Denosumab

Giant Cell Tumor (GCT) of bone is characterized by neoplastic mononuclear stromal cells that express RANKL. This RANKL recruits and activates osteoclast-like multinucleated giant cells, leading to extensive bone resorption. Denosumab is a monoclonal antibody that binds to RANKL, inhibiting this pathway. It is highly effective and FDA-approved for the treatment of GCTs that are unresectable or where surgery would result in severe morbidity.

Question 16

A 7-year-old boy is diagnosed with Legg-Calvé-Perthes disease. According to the Herring Lateral Pillar classification, which radiographic finding places the patient in Group C, indicating a poorer prognosis?

Explanation

Correct Answer: <50% maintenance of lateral pillar height

The Herring Lateral Pillar classification is a key prognostic indicator in Legg-Calvé-Perthes disease, assessed during the fragmentation stage. Group A has no lateral pillar involvement. Group B maintains >50% of lateral pillar height. Group C maintains <50% of lateral pillar height and is associated with a poorer outcome, often requiring surgical containment (e.g., femoral or pelvic osteotomy) in children over 8 years of age.

Question 17

A 55-year-old male with long-standing, poorly controlled diabetes presents with a swollen, erythematous, and warm right foot. Radiographs show fragmentation, destruction, and subluxation of the tarsometatarsal joints. Which of the following best describes the 'neurovascular theory' in the pathogenesis of this condition?

Explanation

Correct Answer: Autonomic neuropathy causes hyperemic bone resorption and weakening

The pathogenesis of Charcot neuroarthropathy involves two main theories. The neurotraumatic theory posits that loss of protective sensation leads to repetitive microtrauma and joint destruction. The neurovascular theory suggests that autonomic neuropathy leads to loss of sympathetic tone, resulting in increased blood flow (hyperemia). This hyperemia causes active bone resorption and osteopenia, making the bone susceptible to fracture and fragmentation even with normal weight-bearing.

Question 18

A 68-year-old man presents with increasing hat size, hearing loss, and bowing of his tibiae. Laboratory tests show markedly elevated serum alkaline phosphatase with normal calcium and phosphorus. Histology of the affected bone would most likely show which of the following pathognomonic features?

Explanation

Correct Answer: Woven bone with a mosaic pattern of prominent cement lines

The patient's presentation is classic for Paget's disease of bone (osteitis deformans). It is characterized by excessive, disorganized bone remodeling. The pathognomonic histologic finding is a 'mosaic' or 'jigsaw puzzle' pattern of lamellar bone with prominent cement lines. This pattern represents haphazard areas of bone resorption by overactive osteoclasts followed by rapid, disorganized bone formation by osteoblasts.

Question 19

A 6-week-old female infant undergoes a screening ultrasound for Developmental Dysplasia of the Hip (DDH) due to a breech presentation. The ultrasound report notes a Graf Type IIa hip. What does this classification indicate, and what is the standard management?

Explanation

Correct Answer: Physiologically immature hip; follow-up ultrasound in 4-6 weeks

In the Graf classification for DDH ultrasound, a Type I hip is normal (alpha angle >60 degrees). Type IIa indicates a physiologically immature hip in an infant less than 3 months old (alpha angle 50-59 degrees). The standard management is observation and a follow-up ultrasound in 4-6 weeks, as the vast majority will mature normally without intervention. Type IIb is the same angle but in an infant >3 months, which requires treatment.

Question 20

A 45-year-old female runner complains of burning pain and tingling in the plantar aspect of her foot, which worsens with activity. Examination reveals a positive Tinel's sign posterior to the medial malleolus. During surgical release of the flexor retinaculum for this condition, which of the following structures is located immediately anterior to the tibial nerve within the tarsal tunnel?

Explanation

Correct Answer: Posterior tibial artery

The patient has Tarsal Tunnel Syndrome. The structures passing through the tarsal tunnel from anterior to posterior (or medial to lateral) are: Tibialis posterior tendon, Flexor Digitorum longus tendon, posterior tibial Artery, tibial Nerve, and Flexor Hallucis longus tendon. This is often remembered by the mnemonic 'Tom, Dick, And Very Nervous Harry'. Therefore, the posterior tibial artery is located immediately anterior to the tibial nerve.

Question 21

A 4-year-old boy presents with short stature, frontal bossing, and rhizomelic shortening of the limbs. Radiographs show narrowing of the interpedicular distances in the lumbar spine. Which of the following is the most likely genetic mechanism responsible for this condition?

Explanation

Correct Answer: C

Achondroplasia (Index 1.2.1) is the most common form of short-limb dwarfism. It is caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, which abnormally inhibits chondrocyte proliferation in the proliferative zone of the physis. A defect in type I collagen is seen in Osteogenesis Imperfecta. COMP mutations are associated with Multiple Epiphyseal Dysplasia and Pseudoachondroplasia. CBFA1 mutations cause Cleidocranial Dysplasia.

Question 22

A 24-year-old female presents with anterior chest wall pain, palmoplantar pustulosis, and severe acne. Radiographs of the sternoclavicular joint reveal hyperostosis and osteitis. Which of the following is the most appropriate initial pharmacological treatment for her osteoarticular symptoms?

Explanation

Correct Answer: B

The patient's presentation is classic for SAPHO syndrome (Synovitis, Acne, Pustulosis, Hyperostosis, Osteitis), which is classified under Osteoarticular Skin Syndromes (Index 2.1.5). It is an inflammatory, non-infectious condition. The first-line treatment for the osteoarticular manifestations is NSAIDs. Antibiotics and surgical debridement are not indicated as the primary treatment since the bone lesions are typically sterile.

Question 23

A 16-year-old boy complains of severe, aching pain in his right thigh that is worse at night and dramatically relieved by ibuprofen. CT scan reveals a 1 cm radiolucent nidus surrounded by dense reactive sclerosis in the proximal femoral diaphysis. What is the primary biochemical mediator responsible for the characteristic pain pattern in this lesion?

Explanation

Correct Answer: C

Osteoid osteoma (Index 8.1.1) is a benign bone-forming tumor characterized by a small nidus (typically less than 1.5 cm). The nidus produces high levels of Prostaglandin E2 (PGE2) and cyclooxygenase-2 (COX-2). This high concentration of PGE2 causes the intense, characteristic nocturnal pain that is highly responsive to NSAIDs.

Question 24

A 13-year-old obese boy presents with a 3-week history of left groin pain and a limp. On physical examination, as the left hip is passively flexed, it obligatorily deviates into external rotation. Radiographs show that a line drawn along the superior border of the femoral neck does not intersect the lateral portion of the femoral epiphysis. What is the most likely diagnosis?

Explanation

Correct Answer: B

The clinical presentation of an obese adolescent with groin pain, obligatory external rotation during hip flexion (Drehmann sign), and a radiograph showing failure of Klein's line to intersect the epiphysis is pathognomonic for Slipped Capital Femoral Epiphysis (SCFE) (Index 13.3).

Question 25

In the evaluation of a 7-year-old boy with Legg-Calvé-Perthes disease, the treating orthopedic surgeon utilizes the Herring lateral pillar classification to determine prognosis. During which stage of the disease is this classification system most accurately applied?

Explanation

Correct Answer: B

The Herring lateral pillar classification is used to predict the outcome in Legg-Calvé-Perthes disease (Index 13.2.1). It is based on the height of the lateral pillar of the capital femoral epiphysis on an AP radiograph. It is most accurately applied during the early fragmentation stage of the disease. Applying it too early (in the initial stage) may underestimate the extent of lateral pillar involvement and lead to an inaccurate prognosis.

Question 26

A 55-year-old female with poorly controlled diabetes mellitus presents with severe global restriction of active and passive shoulder motion. She is diagnosed with idiopathic frozen shoulder. Contracture of which of the following structures is considered the primary anatomical restraint to external rotation in the early stages of this condition?

Explanation

Correct Answer: B

In frozen shoulder or adhesive capsulitis (Index 15.3.4), the earliest and most profound motion loss is typically external rotation with the arm at the side. This is primarily due to contracture, thickening, and fibroplasia of the coracohumeral ligament and the structures within the rotator interval.

Question 27

A 45-year-old typist is scheduled for an open carpal tunnel release. During the surgical approach, the surgeon must be mindful of the contents of the carpal tunnel. Which of the following tendons is located within the carpal tunnel?

Explanation

Correct Answer: B

The carpal tunnel (Index 17.7.1) contains 10 structures: the median nerve, four tendons of the flexor digitorum superficialis, four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis longus. The flexor carpi radialis runs in its own fibro-osseous tunnel within the transverse carpal ligament. The palmaris longus and flexor carpi ulnaris are superficial to the carpal tunnel.

Question 28

A 42-year-old marathon runner presents with burning pain and tingling in the plantar aspect of his right foot, which worsens with prolonged standing. Tinel's sign is positive posterior to the medial malleolus. Which of the following structures forms the roof of the anatomical tunnel implicated in this patient's condition?

Explanation

Correct Answer: C

The patient has Tarsal Tunnel Syndrome (Index 20.5.1), which is a compression neuropathy of the tibial nerve or its branches posterior to the medial malleolus. The roof of the tarsal tunnel is formed by the flexor retinaculum (also known as the laciniate ligament). The floor is formed by the medial surface of the talus, sustentaculum tali, and medial calcaneal wall.

Question 29

A 30-year-old female presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femoral epiphysis extending to the subchondral bone without a sclerotic rim. Biopsy shows mononuclear stromal cells and multinucleated giant cells. Which of the following targeted therapies is most appropriate for advanced or unresectable cases of this tumor?

Explanation

Correct Answer: B

The clinical and radiographic presentation is classic for a Giant Cell Tumor of Bone (GCT) (Index 8.1.8). The neoplastic mononuclear stromal cells express RANKL, which stimulates the recruitment and activation of osteoclast-like multinucleated giant cells, leading to bone resorption. Denosumab, a monoclonal antibody against RANKL, is highly effective and FDA-approved for the treatment of unresectable or recurrent GCT.

Question 30

A 6-year-old girl is evaluated for a short neck, low posterior hairline, and severely restricted cervical range of motion. Radiographs confirm congenital fusion of multiple cervical vertebrae. Which of the following organ systems must be routinely evaluated due to a high association with this syndrome?

Explanation

Correct Answer: B

The patient has Klippel-Feil Syndrome (Index 14.1.1), characterized by the classic triad of a short neck, low posterior hairline, and limited neck motion due to congenital fusion of cervical vertebrae. Up to 30-50% of these patients have associated genitourinary anomalies (e.g., unilateral renal agenesis, horseshoe kidney), making routine renal ultrasound a mandatory part of their initial evaluation.

Question 31

A 4-year-old boy presents with short stature, rhizomelic shortening of the limbs, frontal bossing, and a trident hand. Radiographs show narrowing of the interpedicular distances in the lumbar spine. Which of the following is the most likely genetic mutation responsible for this condition?

Explanation

Correct Answer: B (FGFR3)

The clinical picture describes Achondroplasia, the most common form of short-limb dwarfism. It is an autosomal dominant condition caused by an activating mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, which inhibits chondrocyte proliferation in the proliferative zone of the physis. COL1A1 is associated with Osteogenesis Imperfecta, COMP with Multiple Epiphyseal Dysplasia, RUNX2 with Cleidocranial Dysplasia, and EXT1 with Multiple Hereditary Exostoses.

Question 32

A 12-year-old girl presents with delayed eruption of secondary teeth and the ability to appose her shoulders anteriorly. Radiographs reveal absent clavicles and delayed ossification of the pubic symphysis. A mutation in which of the following transcription factors is most likely responsible?

Explanation

Correct Answer: B (RUNX2 (CBFA1))

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (also known as CBFA1) gene. This gene is a master transcription factor essential for osteoblast differentiation and bone formation. The condition is characterized by hypoplastic or absent clavicles, delayed closure of cranial sutures, and dental anomalies.

Question 33

A 9-year-old girl presents with recurrent episodes of multifocal bone pain and swelling, primarily affecting the clavicle and distal tibia. Laboratory tests show a mildly elevated ESR but normal WBC count. Blood cultures are negative, and a bone biopsy reveals sterile chronic inflammation. She also has palmoplantar pustulosis. What is the most appropriate initial pharmacological treatment?

Explanation

Correct Answer: C (Nonsteroidal anti-inflammatory drugs (NSAIDs))

The clinical picture describes Chronic Recurrent Multifocal Osteomyelitis (CRMO), often associated with SAPHO syndrome (Synovitis, Acne, Pustulosis, Hyperostosis, Osteitis). It is an autoinflammatory condition, not an infectious one. The first-line treatment is NSAIDs. Antibiotics and surgical debridement are not indicated as the lesions are sterile.

Question 34

A 35-year-old male presents with chronic, painless swelling of his right knee. MRI reveals a joint effusion with nodular synovial proliferation that demonstrates low signal intensity on both T1 and T2-weighted images, with a 'blooming' artifact on gradient-echo sequences. What is the characteristic histological finding of this condition?

Explanation

Correct Answer: B (Multinucleated giant cells and hemosiderin-laden macrophages)

The MRI findings of low T1/T2 signal and blooming artifact are classic for Pigmented Villonodular Synovitis (PVNS), due to extensive hemosiderin deposition. Histologically, PVNS is characterized by a proliferation of mononuclear cells, multinucleated giant cells, foam cells, and abundant hemosiderin-laden macrophages.

Question 35

In a patient with a Slipped Capital Femoral Epiphysis (SCFE), the proximal femoral epiphysis typically displaces in which direction relative to the femoral neck?

Explanation

Correct Answer: C (Posterior and inferior)

In SCFE, the femoral neck displaces anteriorly and superiorly relative to the epiphysis. Therefore, the epiphysis is displaced posteriorly and inferiorly relative to the femoral neck. This biomechanical relationship dictates the typical external rotation deformity and obligate external rotation with hip flexion seen clinically.

Question 36

A 19-year-old male complains of severe, aching pain in his right thigh that is worse at night and dramatically relieved by ibuprofen. Radiographs show a small radiolucent nidus surrounded by dense reactive cortical sclerosis in the proximal femoral diaphysis. The dramatic pain relief from NSAIDs is primarily due to the high concentration of which substance within the nidus?

Explanation

Correct Answer: C (Prostaglandin E2)

Osteoid osteoma is a benign bone-forming tumor characterized by a small radiolucent nidus (less than 1.5 cm) surrounded by reactive sclerosis. The nidus produces high levels of Prostaglandin E2 (PGE2), which causes the characteristic night pain that is exquisitely sensitive to NSAIDs or aspirin.

Question 37

A 6-year-old girl is diagnosed with Klippel-Feil syndrome based on a short neck, low posterior hairline, and limited cervical range of motion. Radiographs confirm congenital fusion of multiple cervical vertebrae. Which of the following organ systems must be routinely evaluated due to a high rate of associated congenital anomalies?

Explanation

Correct Answer: B (Genitourinary)

Klippel-Feil syndrome is characterized by congenital fusion of two or more cervical vertebrae. It is highly associated with other congenital anomalies, most notably genitourinary tract anomalies (up to 30-40%, such as unilateral renal agenesis), cardiovascular anomalies, and Sprengel's deformity. A renal ultrasound is mandatory in these patients.

Question 38

A 4-year-old boy presents with congenital bilateral hallux valgus and short first metatarsals. He recently developed painful, firm soft tissue swellings over his back following a minor fall, which subsequently ossified. A mutation in the ACVR1 gene is suspected. Which of the following interventions is strictly contraindicated in this patient?

Explanation

Correct Answer: C (Intramuscular injections and surgical biopsies)

The patient has Fibrodysplasia Ossificans Progressiva (FOP), an autosomal dominant disorder caused by a mutation in the ACVR1 gene. It is characterized by congenital malformation of the great toes and progressive heterotopic ossification. Any soft tissue trauma, including intramuscular injections, surgical biopsies, or dental blocks, can precipitate explosive heterotopic bone formation and is strictly contraindicated.

Question 39

In the evaluation of a 7-year-old boy with Legg-Calvé-Perthes disease, the Herring Lateral Pillar classification is used to determine prognosis. This classification is based on the radiolucency and height of the lateral pillar of the femoral head during which stage of the disease?

Explanation

Correct Answer: B (Fragmentation stage)

The Herring Lateral Pillar classification is the most widely used prognostic classification for Legg-Calvé-Perthes disease. It evaluates the height of the lateral pillar of the capital femoral epiphysis on an AP pelvic radiograph during the fragmentation stage of the disease, as this is when the maximum extent of epiphyseal collapse is evident.

Question 40

A 45-year-old female presents with burning pain and tingling in the plantar aspect of her foot, which worsens with prolonged standing. Tinel's sign is positive posterior to the medial malleolus. She is diagnosed with Tarsal Tunnel Syndrome. Which of the following structures forms the roof of the tarsal tunnel?

Explanation

Correct Answer: C (Flexor retinaculum (laciniate ligament))

Tarsal tunnel syndrome is a compression neuropathy of the posterior tibial nerve or its branches. The tarsal tunnel is a fibro-osseous space located posterior and inferior to the medial malleolus. Its roof is formed by the flexor retinaculum (also known as the laciniate ligament), while the floor is formed by the medial surfaces of the talus and calcaneus.

Question 41

A 13-year-old obese boy presents with groin pain and an obligate external rotation of the hip during flexion. Radiographs confirm a slipped capital femoral epiphysis (SCFE). Which of the following best describes the anatomical displacement of the femoral neck relative to the capital epiphysis in this condition?

Explanation

Correct Answer: The neck displaces anteriorly and superiorly.

In Slipped Capital Femoral Epiphysis (SCFE), the capital femoral epiphysis remains relatively fixed in the acetabulum while the femoral neck displaces anteriorly and superiorly. This biomechanical shift leads to the classic clinical presentation of obligate external rotation when the hip is flexed.

Question 42

A 19-year-old male presents with severe nocturnal thigh pain that is dramatically relieved by NSAIDs. Imaging reveals a 1 cm radiolucent nidus surrounded by dense reactive sclerosis in the proximal femoral diaphysis. What is the primary mechanism by which NSAIDs relieve pain in this specific pathology?

Explanation

Correct Answer: Inhibition of high levels of prostaglandins (PGE2) produced by the nidus.

Osteoid osteomas are benign bone-forming tumors characterized by a small radiolucent nidus. The nidus produces exceptionally high levels of prostaglandins, particularly PGE2, which mediate the intense, often nocturnal pain. NSAIDs inhibit cyclooxygenase, thereby reducing prostaglandin synthesis and providing dramatic, characteristic pain relief.

Question 43

An 65-year-old man presents with increasing hat size and deep, aching bone pain in his pelvis and right femur. Laboratory tests show markedly elevated serum alkaline phosphatase with normal calcium and phosphorus. Histological examination of the affected bone would most likely reveal:

Explanation

Correct Answer: A mosaic pattern of lamellar bone with prominent cement lines.

The clinical presentation is classic for Paget's disease of bone (Osteitis Deformans). The hallmark histological finding in the mixed or sclerotic phase of Paget's disease is a 'mosaic' or 'jigsaw puzzle' pattern of lamellar bone. This is caused by prominent, haphazard cement lines resulting from repeated, uncoordinated episodes of osteoclastic bone resorption and osteoblastic bone formation.

Question 44

A newborn is diagnosed with achondroplasia. Which of the following best describes the underlying genetic mutation and its effect on bone growth?

Explanation

Correct Answer: An activating mutation in the FGFR3 gene leading to inhibition of chondrocyte proliferation in the proliferative zone.

Achondroplasia is the most common form of short-limb dwarfism. It is caused by an autosomal dominant, gain-of-function (activating) mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. This mutation leads to a paradoxical inhibition of chondrocyte proliferation in the proliferative zone of the physis, resulting in impaired endochondral ossification and rhizomelic shortening of the limbs.

Question 45

In the surgical management of chronic bacterial osteomyelitis of the tibia, the surgeon identifies a segment of necrotic cortical bone that has become separated from the surrounding living bone. What is the correct anatomical term for this necrotic bone segment?

Explanation

Correct Answer: Sequestrum

A sequestrum is a piece of dead (necrotic) bone that has become separated from normal, sound bone during the process of chronic osteomyelitis. An involucrum is a layer of new bone growth outside existing bone. A cloaca is an opening in the involucrum through which pus and sequestra can discharge.

Question 46

During an open carpal tunnel release, the surgeon must be careful to avoid injury to the recurrent motor branch of the median nerve. Which of the following muscles is primarily innervated by this specific branch?

Explanation

Correct Answer: Opponens pollicis

The recurrent motor branch of the median nerve innervates the thenar muscles, which can be remembered by the mnemonic 'OAF': Opponens pollicis, Abductor pollicis brevis, and the superficial head of the Flexor pollicis brevis. The adductor pollicis and first dorsal interosseous are innervated by the deep branch of the ulnar nerve.

Question 47

A 10-year-old girl presents with a painless snapping sensation in her lateral knee during extension. MRI confirms a complete (Wrisberg variant) discoid lateral meniscus. What is the primary anatomical deficiency in the Wrisberg variant of a discoid meniscus?

Explanation

Correct Answer: Absence of the posterior meniscotibial (coronary) ligament.

The Wrisberg variant of a discoid lateral meniscus lacks the normal posterior meniscotibial (coronary) ligament attachments to the tibia. Its only posterior attachment is the meniscofemoral ligament of Wrisberg. This lack of tethering allows the meniscus to subluxate anteriorly during knee extension, causing the classic 'snapping knee' syndrome.

Question 48

A 14-year-old boy presents with recurrent ankle sprains and a rigid flatfoot. On physical examination, there is restricted subtalar motion and spasm of the peroneal muscles. Radiographs reveal an 'anteater nose' sign. Which of the following is the most likely diagnosis?

Explanation

Correct Answer: Calcaneonavicular coalition

The 'anteater nose' sign on an oblique radiograph of the foot is pathognomonic for a calcaneonavicular coalition. It represents an elongated anterior process of the calcaneus approaching the navicular. Talocalcaneal coalitions, another common cause of peroneal spastic flatfoot, typically present with the 'C-sign' on lateral radiographs.

Question 49

A 15-year-old female gymnast presents with chronic low back pain. Radiographs demonstrate a grade II anterior translation of L5 on S1 with an identifiable defect in the pars interarticularis. According to the Wiltse classification, what type of spondylolisthesis does this patient have?

Explanation

Correct Answer: Type II (Isthmic)

Type II (Isthmic) spondylolisthesis involves a defect in the pars interarticularis (spondylolysis). It is the most common type in adolescents and athletes, such as gymnasts, who undergo repetitive hyperextension. Type I is dysplastic, Type III is degenerative (common in older adults), Type IV is traumatic (fracture other than the pars), and Type V is pathologic.

Question 50

A 4-year-old child presents with painful soft tissue swellings over the back and neck following minor trauma. Clinical examination reveals a congenital malformation of the great toes (hallux valgus and microdactyly). Radiographs show early heterotopic ossification in the paraspinal muscles. A mutation in which of the following genes is most likely responsible for this condition?

Explanation

Correct Answer: ACVR1

The clinical picture is classic for Fibrodysplasia Ossificans Progressiva (FOP), characterized by congenital malformation of the great toes and progressive heterotopic ossification of skeletal muscles. It is caused by an autosomal dominant mutation in the ACVR1 gene, which encodes a bone morphogenetic protein (BMP) type I receptor. Surgical excision or biopsy of the lesions is strictly contraindicated as it triggers explosive new bone formation.

Question 51

During open reduction of Developmental Dysplasia of the Hip (DDH) via an anterior approach, several anatomical structures must be addressed to allow concentric reduction. Which of the following is NOT typically considered an anatomical block to reduction in DDH?

Explanation

Obstacles to reduction in DDH include the inverted limbus, hypertrophied ligamentum teres, pulvinar, tight iliopsoas tendon, and transverse acetabular ligament. The piriformis is located posteriorly and is not typically a block to reduction.

Question 52

A 25-year-old female presents with a painless mass behind her knee. Radiographs reveal a densely ossified, lobulated mass arising from the posterior cortex of the distal femur without medullary involvement. Biopsy confirms a low-grade spindle cell stroma with well-formed woven bone. Amplification of which of the following genes is most characteristic of this lesion?

Explanation

The clinical and radiographic presentation is classic for parosteal osteosarcoma. This low-grade surface osteosarcoma is characterized by ring chromosomes and amplification of the MDM2 and CDK4 genes on chromosome 12q13-15.

Question 53

A 14-year-old boy presents with a painful scoliosis. Radiographs reveal a right-thoracic scoliotic curve. Advanced imaging shows a 1 cm radiolucent nidus with surrounding sclerosis in the T8 vertebra. Which of the following best describes the most likely location of the lesion and its relationship to the scoliotic curve?

Explanation

Osteoid osteomas of the spine typically arise in the posterior elements (pedicle or lamina). They cause asymmetric muscle spasm, leading to a painful scoliosis with the lesion classically located on the concavity of the curve.

Question 54

An 11-year-old obese male presents to the emergency department unable to bear weight on his left leg after a minor slip. Radiographs confirm a severe left slipped capital femoral epiphysis (SCFE). Which of the following factors is most predictive of developing avascular necrosis (AVN) of the femoral head in this patient?

Explanation

The inability to bear weight defines an unstable SCFE according to the Loder classification. Unstable SCFE has a significantly higher risk of developing AVN (up to 50%) compared to stable SCFE, regardless of slip severity.

Question 55

A 9-year-old boy presents with a diaphyseal permeative lesion of the femur and periosteal "onion-skin" reaction. Histopathology reveals sheets of small, uniform round blue cells. Cytogenetic analysis is most likely to show which of the following chromosomal translocations?

Explanation

The clinical and histological description is classic for Ewing sarcoma. The t(11;22) translocation, creating the EWS-FLI1 fusion protein, is found in approximately 85% of Ewing sarcoma cases.

Question 56

A 12-year-old boy presents with a painful, swollen thigh. Radiographs demonstrate a permeative diaphyseal lesion with a periosteal 'onion-skin' reaction. A core biopsy confirms a small round blue cell tumor. Which of the following cytogenetic abnormalities is most characteristically associated with this diagnosis?

Explanation

Ewing sarcoma is classically associated with the t(11;22)(q24;q12) translocation, resulting in the EWS-FLI1 fusion protein. The t(X;18) translocation is seen in synovial sarcoma, and t(9;22) is associated with chronic myelogenous leukemia.

Question 57

A 5-year-old boy presents with waddling gait and delayed motor milestones. Radiographs show flattened, irregular epiphyses in the hips and knees, but the spine radiographs show normal vertebral bodies. What is the most likely genetic mutation responsible for his condition?

Explanation

The patient has Multiple Epiphyseal Dysplasia (MED), which spares the spine (unlike Spondyloepiphyseal Dysplasia, linked to COL2A1). MED is most commonly caused by mutations in the COMP (Cartilage Oligomeric Matrix Protein) gene.

Question 58

A 6-week-old female is being treated with a Pavlik harness for Developmental Dysplasia of the Hip (DDH). During follow-up, she exhibits decreased active extension of the knee on the affected side. What is the most appropriate next step in management?

Explanation

Decreased active knee extension indicates a femoral nerve palsy, a known complication of excessive hip flexion in a Pavlik harness. The harness should be adjusted to decrease flexion or temporarily removed until nerve function recovers.

Question 59

A 15-year-old girl is diagnosed with conventional high-grade osteosarcoma of the distal femur. She completes neoadjuvant chemotherapy, undergoes wide surgical resection, and the pathology reveals 95% tumor necrosis. What is the most significant prognostic implication of this histologic finding?

Explanation

The percentage of tumor necrosis following neoadjuvant chemotherapy is the most important prognostic factor in osteosarcoma. Greater than 90% necrosis characterizes a good responder and is associated with significantly improved long-term survival.

Question 60

A newborn is evaluated for severe limb shortening, rigid clubfeet, 'hitchhiker' thumbs, and cystic swelling of the external ear (cauliflower ear). Radiographs show short, thick tubular bones. Mutations in which of the following genes are responsible for this syndrome?

Explanation

The clinical picture describes Diastrophic Dysplasia, an autosomal recessive condition. It is caused by mutations in the SLC26A2 gene, which encodes a sulfate transporter (DTDST), leading to impaired cartilage sulfation.

Question 61

A 10-year-old boy with chronic renal failure presents with a limp and obligate external rotation of the hip during flexion. Radiographs confirm a moderate stable slipped capital femoral epiphysis (SCFE). In addition to pinning the affected hip, what is the primary indication for prophylactic pinning of the contralateral hip?

Explanation

Patients with SCFE associated with endocrinopathies or metabolic disorders (like renal failure, hypothyroidism) have a very high risk of bilateral involvement. Prophylactic pinning of the contralateral hip is highly recommended in these specific populations.

Question 62

A 14-year-old boy reports nocturnal thigh pain that is completely relieved by ibuprofen. Imaging shows a 7 mm radiolucent nidus surrounded by reactive sclerosis in the proximal femur cortex. Which inflammatory mediator is characteristically secreted in high levels by this lesion?

Explanation

Osteoid osteomas characteristically secrete high levels of Prostaglandin E2 (PGE2) due to elevated COX-2 expression. This mediates the typical nocturnal pain that dramatically responds to NSAIDs.

Question 63

A 6-year-old boy sustains an extension-type Gartland III supracondylar humerus fracture. On examination, he cannot flex his thumb interphalangeal joint or the distal interphalangeal joint of his index finger. Which structure is most likely injured?

Explanation

The inability to make an 'OK sign' (flexion of thumb IP and index DIP joints) indicates an injury to the anterior interosseous nerve (AIN). AIN palsy is the most common neurologic deficit associated with extension-type supracondylar humerus fractures.

Question 64

A 12-year-old girl is noted to have a large head with delayed closure of fontanelles, dental anomalies, and the ability to appose her shoulders anteriorly. Radiographs show absent clavicles. What is the function of the protein encoded by the gene mutated in this condition?

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (CBFA1), which is a master transcription factor essential for osteoblast differentiation. This defect impairs normal intramembranous ossification.

Question 65

A 16-year-old boy presents with knee pain. Radiographs reveal a well-circumscribed lytic lesion in the distal femoral epiphysis with central calcifications. Histology shows mononuclear cells, osteoclast-like giant cells, and 'chicken-wire' calcification. What is the most appropriate initial treatment?

Explanation

The diagnosis is chondroblastoma, a benign but locally aggressive epiphyseal tumor characterized by 'chicken-wire' calcifications. The standard of care is aggressive intralesional curettage, high-speed burring, and bone grafting.

Question 66

During the Ponseti method of casting for a newborn with idiopathic clubfoot, the sequence of correction is critical. After correcting the cavus deformity by elevating the first ray, what is the next step in the manipulative correction process?

Explanation

In the Ponseti method (CAVE acronym), Cavus is corrected first. The next step is simultaneous correction of Adduction and Varus by abducting the forefoot around the fixed head of the talus, leaving Equinus for last.

Question 67

A 32-year-old female presents with a destructive, expansile lytic lesion in the proximal tibia extending to the subchondral bone. Biopsy confirms Giant Cell Tumor of bone. She is treated with denosumab preoperatively. What is the specific molecular target of denosumab?

Explanation

Denosumab is a monoclonal antibody that specifically binds to and inhibits RANKL. In Giant Cell Tumors, the neoplastic stromal cells express high levels of RANKL, which recruits and activates the destructive osteoclast-like giant cells.

Question 68

A 9-year-old girl presents with a limp, precocious puberty, and large irregular cafe-au-lait spots on her torso. Radiographs show a 'ground-glass' expansile lesion in her proximal femur with a 'shepherd's crook' deformity. What is the underlying pathophysiology of her bony lesions?

Explanation

McCune-Albright syndrome is characterized by polyostotic fibrous dysplasia, cafe-au-lait spots, and endocrinopathies. It is caused by a post-zygotic activating somatic mutation in the GNAS1 gene, resulting in continuous cAMP production.

Question 69

A 7-year-old boy is diagnosed with Legg-Calve-Perthes disease. According to the lateral pillar (Herring) classification, which radiographic feature in the fragmentation stage portends the worst prognosis?

Explanation

The Herring Lateral Pillar classification predicts the outcome of Legg-Calve-Perthes disease based on the height of the lateral pillar during the fragmentation stage. Group C (<50% lateral pillar height maintained) indicates severe collapse and carries the worst prognosis.

Question 70

A 25-year-old male presents with a slowly enlarging, painful mass near the knee joint, but entirely extra-articular. MRI shows a soft tissue mass with focal calcifications. Biopsy shows a biphasic pattern of epithelial and spindle cells. Which specific genetic translocation confirms this diagnosis?

Explanation

Synovial sarcoma classically presents as a slow-growing calcified mass near a joint in young adults and frequently exhibits a biphasic histology. It is definitively identified by the t(X;18)(p11;q11) translocation forming the SYT-SSX fusion gene.

Question 71

A 12-year-old boy presents with progressive, severe mid-thigh pain and swelling. Radiographs reveal a poorly marginated, permeative lytic lesion in the femoral diaphysis with an 'onion-skin' periosteal reaction. Biopsy demonstrates uniform small round blue cells that stain strongly positive for CD99. Which of the following chromosomal translocations is most characteristic of this diagnosis?

Explanation

Ewing sarcoma typically presents as a diaphyseal permeative lesion with onion-skinning and CD99+ small round blue cells. The characteristic genetic abnormality is the t(11;22) translocation involving the EWSR1 and FLI1 genes.

Question 72

An 8-year-old boy is evaluated for a waddling gait and knee pain. He has normal spine radiographs with no evidence of platyspondyly, but appendicular skeleton films show irregular, delayed ossification of multiple epiphyses and a characteristic 'double-layered' patella. A mutation in which of the following genes is most likely responsible for his condition?

Explanation

Multiple epiphyseal dysplasia (MED) is most commonly caused by an autosomal dominant mutation in the Cartilage Oligomeric Matrix Protein (COMP) gene. A double-layered patella is a classic radiographic hallmark that helps differentiate MED from other skeletal dysplasias.

Question 73

A 15-year-old boy presents with chronic right knee pain. Radiographs reveal a well-circumscribed, eccentrically located lytic lesion in the proximal tibial epiphysis with stippled calcifications. Histological examination of the curetted lesion shows mononuclear cells with grooved nuclei and areas of 'chicken-wire' pericellular calcification. What is the most likely diagnosis?

Explanation

Chondroblastoma is a rare, benign cartilage-forming tumor that classically arises in the epiphysis or apophysis of skeletally immature patients. Histology is distinguished by chondroblasts with grooved nuclei and a fine network of 'chicken-wire' calcifications.

Question 74

A 3-month-old female with developmental dysplasia of the hip (DDH) has been treated in a Pavlik harness for two weeks. Her mother reports that the baby is no longer kicking her right leg. Examination reveals absent active knee extension on the right, but ankle and toe movements are normal. What is the most likely cause of this finding?

Explanation

Femoral nerve palsy is the most common neurologic complication associated with the Pavlik harness, typically resulting from hyperflexion of the hip. Management consists of adjusting the harness to decrease flexion, which usually allows the palsy to resolve completely.

Question 75

A neonate presents with severe rhizomelic limb shortening, rigid clubfeet, 'hitchhiker' thumbs, and cystic swelling of the external ears (cauliflower ears). Which of the following gene mutations is the underlying cause of this skeletal dysplasia?

Explanation

Diastrophic dysplasia is caused by an autosomal recessive mutation in the SLC26A2 (DTDST) gene, which encodes a sulfate transporter. Classic clinical features include hitchhiker thumbs, severe rigid clubfeet, and cauliflower ears.

Question 76

A 12-year-old boy with a BMI of 35 is diagnosed with a severe left slipped capital femoral epiphysis (SCFE) and undergoes in situ pinning. In which of the following scenarios is prophylactic pinning of the contralateral asymptomatic hip most strongly indicated?

Explanation

Prophylactic pinning of the contralateral hip in SCFE is highly recommended for patients with endocrine disorders (such as hypothyroidism) due to a very high risk of bilateral involvement. Patients who present at an atypically young age (less than 10 years) are also strong candidates for prophylaxis.

Question 77

A 16-year-old male complains of nocturnal anterior thigh pain that is completely relieved by ibuprofen. Imaging shows a 1 cm radiolucent nidus with intense surrounding reactive sclerosis in the medial femoral diaphysis. What is the preferred minimally invasive definitive treatment?

Explanation

Osteoid osteomas typically present with night pain relieved by NSAIDs and demonstrate a characteristic radiolucent nidus smaller than 1.5 cm. CT-guided radiofrequency ablation (RFA) is the current standard of care for definitive, minimally invasive treatment.

Question 78

A 3-year-old obese girl presents with progressive bilateral tibia vara. Radiographs demonstrate prominent metaphyseal beaking of the medial proximal tibias with a metaphyseal-diaphyseal angle of 20 degrees. Based on her age and radiographic findings, what is the most appropriate initial management?

Explanation

Infantile Blount disease (age under 4 years) with Langenskiold stage I or II and an elevated metaphyseal-diaphyseal angle should initially be treated conservatively. KAFO bracing is the first-line treatment to unload the medial compartment and guide normal growth.

Question 79

A 14-year-old girl is evaluated for right shoulder pain. Radiographs show an expansile, eccentric, lucent lesion in the proximal humeral metaphysis. MRI demonstrates multiple internal fluid-fluid levels. Biopsy confirms blood-filled cystic spaces without an endothelial lining. Which genetic translocation is highly characteristic of this primary lesion?

Explanation

Primary aneurysmal bone cysts (ABCs) are benign but locally aggressive neoplastic processes driven by a t(16;17) translocation that upregulates the USP6 oncogene. Imaging classically shows an expansile metaphyseal lesion with fluid-fluid levels on MRI.

Question 80

A 2-year-old boy presents with marked anterolateral bowing of his left tibia. Physical examination reveals six axillary freckles and multiple cafe-au-lait spots measuring greater than 5 mm. Radiographs show medullary narrowing and sclerosis at the apex of the bowing. What is the expected natural history of this orthopedic condition if left untreated?

Explanation

Anterolateral bowing of the tibia is highly associated with Neurofibromatosis type 1 (NF1). If left untreated, the dysplastic bone characteristically progresses to a spontaneous fracture that fails to heal, resulting in congenital pseudarthrosis of the tibia.

Question 81

A 14-year-old boy presents with a painful mass in his left mid-thigh. Radiographs show a destructive diaphyseal lesion with an "onion-skin" periosteal reaction. Biopsy reveals sheets of small, round, blue cells. Which of the following cytogenetic abnormalities is most characteristically associated with this condition?

Explanation

Ewing sarcoma classically presents as a diaphyseal lesion with onion-skinning and small round blue cells. The pathognomonic translocation is t(11;22)(q24;q12), which creates the EWSR1-FLI1 fusion gene.

Question 82

A 10-year-old girl is evaluated for multiple painless bony prominences around her knees and shoulders. Radiographs demonstrate multiple sessile and pedunculated lesions growing away from the physes. Genetic testing reveals an EXT1 mutation. The pathophysiology of her condition is directly related to a defect in the synthesis of which of the following?

Explanation

Multiple hereditary exostoses (MHE) is caused by mutations in the EXT1 or EXT2 genes. These genes encode glycosyltransferases essential for the synthesis of heparan sulfate, a critical regulator of chondrocyte proliferation.

Question 83

A 12-year-old obese boy presents with a 4-week history of right groin pain and a limp. He denies trauma. During the physical examination of the right hip, what is the most characteristic finding indicating a slipped capital femoral epiphysis (SCFE)?

Explanation

The Drehmann sign is the classic physical exam finding in SCFE. As the hip is passively flexed, the thigh obligatorily abducts and externally rotates due to the altered geometry of the proximal femur.

Question 84

An 8-year-old child presents with delayed eruption of secondary teeth and an unusual ability to bring both shoulders together anteriorly. Radiographs demonstrate bilateral absent clavicles and coxa vara. A mutation in which of the following transcription factors is responsible for this condition?

Explanation

Cleidocranial dysplasia presents with hypoplastic or absent clavicles, delayed fontanelle closure, and dental anomalies. It is inherited in an autosomal dominant pattern due to mutations in the RUNX2 (CBFA1) gene, which controls osteoblast differentiation.

Question 85

A 4-month-old infant with developmental dysplasia of the hip (DDH) is being treated with a Pavlik harness. During a follow-up visit, the parents report the infant is not kicking the affected leg. Examination reveals decreased active knee extension on that side. This complication is most likely due to which of the following mechanical issues?

Explanation

Femoral nerve palsy is a known complication of the Pavlik harness, typically caused by hyperflexion of the hip. Excessive hip abduction, on the other hand, increases the risk of avascular necrosis (AVN) of the femoral head.

Question 86

A 16-year-old boy presents with aching pain in his right anterior tibia that worsens at night. The pain is completely relieved by ibuprofen. A CT scan reveals a 0.8 cm radiolucent nidus surrounded by dense sclerotic bone. The intense pain experienced by this patient is primarily mediated by high local concentrations of which substance?

Explanation

Osteoid osteomas secrete high levels of prostaglandins, particularly PGE2, which cause intense local vasodilation and pain, classically at night. This is why the pain is typically highly responsive to NSAIDs.

Question 87

A newborn presents with severe micromelic short stature, "hitchhiker" thumbs, clubfeet, and "cauliflower" ears. Genetic analysis reveals a mutation in the SLC26A2 gene. Which of the following best describes the underlying biochemical defect in this dysplasia?

Explanation

Diastrophic dysplasia is caused by a mutation in SLC26A2, a sulfate transporter gene. The resulting intracellular sulfate deficiency leads to undersulfation of proteoglycans in the cartilage matrix.

Question 88

A 6-year-old boy is diagnosed with Legg-Calve-Perthes disease. According to the Herring lateral pillar classification, which of the following radiographic parameters best determines his prognosis and likelihood of developing femoral head deformity?

Explanation

The Herring classification focuses on the height of the lateral third (lateral pillar) of the capital femoral epiphysis during the fragmentation phase. A maintained lateral pillar height (>50%) correlates with a more spherical femoral head outcome.

Question 89

A 15-year-old boy presents with shoulder pain. Radiographs reveal a radiolucent, eccentrically located epiphyseal lesion in the proximal humerus with a thin sclerotic margin. Biopsy shows polygonal chondroblasts and characteristic "chicken-wire" calcification. What is the most likely diagnosis?

Explanation

Chondroblastomas are benign cartilage tumors that classically arise in the epiphyses of long bones in skeletally immature patients. Histology typically reveals chondroblasts, giant cells, and fine, pericellular "chicken-wire" calcifications.

Question 90

A newborn is diagnosed with idiopathic clubfoot (talipes equinovarus) and treatment is initiated using the Ponseti method. What is the correct initial step in manipulating the foot prior to applying the first cast?

Explanation

The Ponseti method requires a specific sequence of correction (CAVE: Cavus, Adductus, Varus, Equinus). The first step is to supinate the forefoot and elevate the first ray to align the forefoot with the hindfoot, correcting the cavus.

Question 91

A 5-year-old girl is evaluated for disproportionate short stature. Physical exam reveals a barrel chest, severe myopia, and normal intelligence. Radiographs show delayed ossification of the pubic bones, coxa vara, and platyspondyly. Which of the following genes is most likely mutated?

Explanation

Spondyloepiphyseal dysplasia congenita (SEDC) is caused by mutations in the COL2A1 gene, affecting type II collagen. It classically presents with disproportionate short trunk dwarfism, myopia, and atlantoaxial instability.

Question 92

A 26-year-old woman presents with knee pain. Imaging reveals an eccentric, expansile lytic lesion in the distal femoral epiphysis extending into the metaphysis, without a sclerotic rim. Biopsy confirms a giant cell tumor of bone. If medical therapy is considered for an unresectable lesion, which mechanism of action is targeted by the drug of choice?

Explanation

Denosumab is an effective medical treatment for unresectable giant cell tumors of bone. It is a monoclonal antibody that binds to RANKL, preventing it from activating RANK receptors on the multinucleated giant cells (which are osteoclast-like).

Question 93

A 3-year-old severely obese girl presents with bilateral bowing of her legs and a lateral thrust during the stance phase of gait. Standing radiographs show significant tibia vara. Which radiographic parameter definitively distinguishes infantile Blount disease from physiologic bowing?

Explanation

A metaphyseal-diaphyseal angle (Drennan's angle) greater than 16 degrees strongly suggests infantile Blount disease rather than physiologic bowing, which typically has an angle of less than 10 degrees.

Question 94

A 12-year-old boy has an incidental finding on a knee radiograph taken for a sprain. The x-ray shows a 3 cm, eccentrically located, sharply demarcated, multi-loculated radiolucent lesion with a sclerotic rim in the distal tibial metaphysis. If a biopsy were performed, what would be the most likely histologic finding?

Explanation

The clinical and radiographic picture is classic for a non-ossifying fibroma (NOF). Histologically, NOFs consist of spindle-shaped fibroblasts arranged in a whorled or storiform pattern, admixed with multinucleated giant cells and foamy histiocytes.

Question 95

A 9-year-old girl is evaluated for a leg length discrepancy and a limp. Radiographs demonstrate a "shepherd's crook" deformity of the proximal femur with a "ground-glass" appearance in the medullary canal. Physical examination reveals large café-au-lait spots with irregular borders. Which of the following genetic alterations is responsible for this condition?

Explanation

The patient has McCune-Albright syndrome, characterized by polyostotic fibrous dysplasia, café-au-lait spots (coast of Maine), and endocrine abnormalities. It is caused by an activating post-zygotic somatic missense mutation in the GNAS gene, leading to increased cAMP.

Question 96

A 5-year-old boy falls from monkey bars and sustains a significantly displaced extension-type supracondylar humerus fracture. On evaluation in the emergency department, his hand is warm and pink, but the radial pulse is absent. What is the most appropriate next step in management?

Explanation

In a "pink, pulseless" hand associated with a supracondylar humerus fracture, the collateral circulation is intact. The most appropriate next step is urgent closed reduction and percutaneous pinning, which frequently restores the radial pulse by relieving arterial kinking.

Question 97

An 8-year-old boy presents to the emergency department after a minor fall with arm pain. Radiographs reveal a pathologic fracture through a central, radiolucent, cystic lesion in the proximal humerus metaphysis. A piece of cortical bone is seen resting at the bottom of the cyst. If this lesion were aspirated prior to the fracture, what type of fluid would most likely be obtained?

Explanation

The "fallen leaf" sign is pathognomonic for a pathologic fracture through a unicameral bone cyst (UBC). Aspiration of an intact UBC typically yields clear, straw-colored serous fluid that is high in prostaglandins and alkaline phosphatase.

Question 98

A 6-year-old child presents with severe kyphoscoliosis, knock-knees, and normal intelligence. Radiographs show anterior central beaking of the vertebral bodies and severe atlantoaxial instability. Urinalysis reveals excess keratan sulfate. This patient's condition is due to a deficiency of which of the following enzymes?

Explanation

Morquio syndrome (Mucopolysaccharidosis Type IV) is characterized by normal intelligence, severe skeletal deformities, atlantoaxial instability, and keratan sulfate in the urine. It is caused by a deficiency in N-acetylgalactosamine-6-sulfatase (Type IVA) or beta-galactosidase (Type IVB).

Question 99

A 13-year-old boy sustains an ankle injury while skateboarding. Radiographs show a Salter-Harris III fracture of the anterolateral aspect of the distal tibial epiphysis. The mechanism of injury for this specific fracture is most commonly related to which of the following forces?

Explanation

A Juvenile Tillaux fracture is a Salter-Harris III fracture of the anterolateral distal tibial epiphysis. It is caused by an external rotation force that pulls on the anterior inferior tibiofibular ligament (AITFL) as the medial and central physes close earlier than the lateral portion.

Question 100

A 15-year-old girl presents with a rapidly expanding, painful mass in her proximal tibia. MRI reveals an eccentric, metaphyseal lesion with multiple fluid-fluid levels. Biopsy confirms an aneurysmal bone cyst (ABC). Primary ABCs are most frequently driven by a specific genetic rearrangement involving which of the following genes?

Explanation

Primary aneurysmal bone cysts (ABCs) are now recognized as genuine neoplasms driven by characteristic cytogenetic translocations, most commonly t(16;17), resulting in the upregulation of the USP6 gene.

Pediatric Orthopaedics: Comprehensive MCQ Question Bank & Exam Prep

Orthopedic Surgery MCQs: Dysplasias, Tumors & Pediatrics

Orthopedic Surgery MCQs: Dysplasias, Tumors & Pediatrics

Chapter Index