ABOS Board Review: Orthopedic Pathology | Part 12

Question 1

A 68-year-old male presents with generalized fatigue, worsening back pain, and a recent humeral shaft fracture after a minor fall. Radiographs demonstrate multiple punched-out lytic lesions.

Which of the following factors is primarily responsible for the suppression of osteoblastic activity in this patient's disease process?

Explanation

This patient has multiple myeloma, characterized by punched-out lytic lesions. Myeloma cells secrete DKK-1, which inhibits the Wnt signaling pathway, thereby suppressing osteoblast differentiation and activity.

Question 2

A 9-year-old boy presents with severe leg pain, easy fatigability, and a waddling gait. Radiographs reveal symmetrical, bilateral cortical thickening of the diaphyseal regions of the femur and tibia, sparing the epiphyses.

What is the genetic mutation associated with this condition?

Explanation

The clinical and radiographic presentation is classic for Camurati-Engelmann disease (progressive diaphyseal dysplasia). It is an autosomal dominant condition caused by mutations in the TGFB1 gene, leading to increased bone formation in the diaphyses.

Question 3

A 45-year-old female is 8 weeks status post closed reduction and cast immobilization for a distal radius fracture. She complains of severe, burning pain that is disproportionate to the injury, along with skin color changes and stiffness.

Radiographs show patchy osteopenia. Which intervention has been shown to reduce the incidence of this condition when administered daily following a distal radius fracture?

Explanation

This patient has Complex Regional Pain Syndrome (CRPS) or Sudeck's atrophy. Daily administration of 500 mg of Vitamin C for 50 days after a distal radius fracture has been shown to significantly decrease the risk of developing CRPS.

Question 4

Which of the following describes the earliest biochemical change occurring in the articular cartilage during the pathogenesis of osteoarthritis?

Explanation

The earliest biochemical change in osteoarthritis is an increase in the water content of the articular cartilage. This is accompanied by a decrease in proteoglycan concentration and an alteration in the collagen network.

Question 5

A 25-year-old female with known rheumatoid arthritis presents with worsening neck pain and myelopathic symptoms. Radiographs of the cervical spine reveal an anterior atlanto-dens interval (ADI) of 11 mm. Which of the following radiographic measurements is the most reliable predictor of neurologic recovery after surgical decompression and stabilization?

Explanation

The Space Available for the Cord (SAC), also known as the posterior atlanto-dens interval (PADI), is the most reliable predictor of neurologic deficit and recovery in RA cervical subluxation. A SAC of less than 14 mm is an indication for surgery.

Question 6

A 35-year-old male undergoes curettage and cementation of a giant cell tumor of the distal femur. Six months later, he develops a massive recurrence that is deemed unresectable. Medical therapy with denosumab is initiated. What is the mechanism of action of this medication?

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL, preventing it from activating RANK on the surface of osteoclasts and their precursors. This effectively halts the bone destruction mediated by the reactive osteoclasts in giant cell tumors.

Question 7

A 14-year-old boy presents with a painful, swollen thigh. Radiographs demonstrate a permeative, diaphyseal lytic lesion with an 'onion-skin' periosteal reaction. A biopsy confirms Ewing sarcoma. Which of the following chromosomal translocations is most characteristic of this tumor?

Explanation

Ewing sarcoma is classically associated with the t(11;22) chromosomal translocation, resulting in the EWS-FLI1 fusion protein. This is present in approximately 85% of cases.

Question 8

A 55-year-old male presents with severe acute monoarticular pain, erythema, and swelling in his first metatarsophalangeal joint. Joint aspiration reveals negatively birefringent, needle-shaped crystals under polarized light microscopy. Which of the following is the composition of these crystals?

Explanation

The patient has an acute attack of gout. Gout is characterized by the deposition of monosodium urate crystals in the joint space, which appear as needle-shaped and negatively birefringent under polarized light microscopy.

Question 9

A 70-year-old male with long-standing Paget disease of bone complains of a sudden increase in pain and swelling in his right proximal femur over the last two months. Radiographs reveal a new, destructive lytic lesion with cortical breakthrough. What is the most likely diagnosis?

Explanation

Secondary osteosarcoma is the most common malignant transformation seen in patients with Paget disease of bone. It occurs in about 1% of patients and portends a very poor prognosis.

Question 10

A 10-month-old infant is evaluated for short stature, frontal bossing, and a waddling gait. Radiographs reveal widening of the physes, cupping of the metaphyses, and bowing of the long bones. Laboratory studies show normal calcium, low phosphorus, and elevated alkaline phosphatase. Which of the following describes the underlying histologic defect?

Explanation

The clinical and radiographic presentation is consistent with rickets. The fundamental histologic defect in rickets (and osteomalacia in adults) is the failure of osteoid mineralization, leading to widened, unmineralized growth plates.

Question 11

A 30-year-old male with known ankylosing spondylitis presents to the emergency department with severe back pain after a low-speed motor vehicle collision. Neurological exam is intact. Radiographs are difficult to interpret due to extensive ossification. What is the most appropriate next step in management?

Explanation

Patients with ankylosing spondylitis are at a high risk for highly unstable, three-column spinal fractures even with minor trauma. A CT scan or MRI of the entire spine is mandatory if a fracture is suspected, as these fractures are easily missed on plain radiographs.

Question 12

A 40-year-old female presents with a swelling in her knee joint. MRI reveals a large joint effusion with synovial hypertrophy demonstrating 'blooming artifact' on gradient-echo sequences. Biopsy confirms pigmented villonodular synovitis (PVNS/TGCT). What is the specific genetic aberration driving the overproliferation of cells in this condition?

Explanation

Pigmented villonodular synovitis (PVNS), or tenosynovial giant cell tumor (TGCT), is driven by a t(1;2) translocation involving the CSF1 gene. This leads to an overexpression of CSF-1, which recruits a large inflammatory infiltrate of macrophages.

Question 13

Which of the following criteria is NOT part of the Mirels' scoring system for predicting the risk of a pathologic fracture in a long bone?

Explanation

The Mirels' scoring system includes four categories: Site (upper limb, lower limb, peritrochanteric), Size (<1/3, 1/3-2/3, >2/3 cortical diameter), Nature (blastic, mixed, lytic), and Pain (mild, moderate, functional). Age and gender are not included.

Question 14

A 15-year-old male presents with bone pain and a palpable mass in his distal femur. Biopsy reveals malignant, spindle-shaped cells producing disorganized osteoid. Genetic testing of the tumor cells identifies a mutation in the TP53 gene. Which familial cancer syndrome is most commonly associated with this genetic profile?

Explanation

Osteosarcoma is strongly associated with Li-Fraumeni syndrome, an autosomal dominant disorder caused by a germline mutation in the TP53 tumor suppressor gene. Patients are at high risk for sarcomas, breast cancer, brain tumors, and leukemia.

Question 15

A 35-year-old male presents with right hip pain. Radiographs reveal a benign-appearing, asymptomatic sessile osteochondroma of the proximal femur that has been stable for 15 years. The patient notes recent rapid growth and increasing pain. What MRI finding is most highly predictive of malignant transformation to a secondary chondrosarcoma?

Explanation

In adults, a cartilage cap thickness of greater than 2 cm on MRI is highly suspicious for malignant transformation of an osteochondroma into a secondary chondrosarcoma. Rapid growth and new onset of pain also raise suspicion.

Question 16

A newborn is diagnosed with a skeletal dysplasia characterized by rhizomelic shortening of the limbs, a large head with frontal bossing, and a midface hypoplasia. The condition is inherited in an autosomal dominant pattern, though 80% are de novo mutations linked to advanced paternal age. What is the fundamental cellular defect?

Explanation

Achondroplasia is caused by a gain-of-function mutation in the FGFR3 gene. FGFR3 normally acts to inhibit cartilage proliferation; thus, a gain-of-function mutation constitutively suppresses enchondral ossification, leading to dwarfism.

Question 17

A 6-year-old girl is evaluated for multiple recurrent long bone fractures with minimal trauma. Examination reveals blue sclerae and mild hearing loss. Radiographs show osteopenia and multiple healed fractures. Which of the following genes is most likely mutated in this patient?

Explanation

This patient has Osteogenesis Imperfecta (OI), primarily characterized by brittle bones, blue sclerae, and hearing loss. Approximately 90% of OI cases are caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes, affecting Type I collagen.

Question 18

A 12-year-old girl presents with precocious puberty, cafe-au-lait spots with irregular 'coast of Maine' borders, and a radiolucent 'ground glass' lesion in her proximal femur causing a shepherd's crook deformity. What is the underlying pathophysiology of her bone disease?

Explanation

This presentation is classic for McCune-Albright syndrome, consisting of polyostotic fibrous dysplasia, cafe-au-lait macules, and endocrine abnormalities. It is caused by a somatic activating mutation in the GNAS1 gene, resulting in elevated intracellular cAMP.

Question 19

A 60-year-old female complains of right knee pain, stiffness, and swelling. Aspiration yields cloudy, low-viscosity fluid. Polarized light microscopy reveals rhomboid-shaped, positively birefringent crystals. This finding is most frequently associated with which of the following metabolic conditions?

Explanation

The presence of positively birefringent, rhomboid-shaped crystals indicates calcium pyrophosphate dihydrate (CPPD) deposition disease (pseudogout). CPPD is strongly associated with hyperparathyroidism, hemochromatosis, hypothyroidism, and hypomagnesemia (the '4 Hs').

Question 20

A patient is evaluated for suspected Complex Regional Pain Syndrome (CRPS) Type I six months after an ankle fracture. According to the Budapest Criteria for the clinical diagnosis of CRPS, the patient must report at least one symptom in three of four categories. Which of the following is NOT one of the designated categories?

Explanation

The Budapest Criteria for CRPS rely entirely on clinical signs and symptoms across four categories: Sensory, Vasomotor, Sudomotor/Edema, and Motor/Trophic. Radiographic findings like osteopenia are supportive but are not part of the formal clinical criteria.

Question 21

A 68-year-old male presents with generalized bone pain, lethargy, and recurrent respiratory infections. Radiographs demonstrate multiple punched-out lytic lesions in the skull and axial skeleton.

What is the most appropriate initial screening laboratory test to establish the underlying diagnosis?

Explanation

This patient's presentation and radiographic findings are classic for multiple myeloma. Serum and urine protein electrophoresis (SPEP/UPEP) with immunofixation are the most sensitive initial screening tests to detect monoclonal immunoglobulins. A skeletal survey is preferred over a Technetium-99m bone scan, which is often falsely negative due to the lack of reactive bone formation in myeloma.

Question 22

A 9-year-old boy presents with severe, aching bilateral leg pain, a waddling gait, and progressive muscle weakness. Radiographs demonstrate marked, symmetrical cortical thickening of the diaphyseal regions of the femurs and tibias.

What is the genetic mutation responsible for this specific condition?

Explanation

The clinical and radiographic presentation is characteristic of Camurati-Engelmann disease (progressive diaphyseal dysplasia). This condition is inherited in an autosomal dominant pattern and is caused by a gain-of-function mutation in the TGFB1 gene. The mutation leads to increased osteoblastic activity, resulting in marked cortical thickening of the long bone diaphyses.

Question 23

A 55-year-old female presents 8 weeks after closed reduction and casting of a distal radius fracture. She reports intense, burning pain, swelling, and vasomotor changes in her hand. Radiographs show patchy periarticular osteopenia.

Which of the following prophylactic measures has been prospectively shown to reduce the incidence of this condition when administered at the time of injury?

Explanation

This patient has developed Complex Regional Pain Syndrome (CRPS), historically known as Sudeck's atrophy. Prophylactic administration of Vitamin C at a dose of 500 mg daily for 50 days following a distal radius fracture has been shown to significantly reduce the risk of developing CRPS. It is believed to work via its antioxidant properties.

Question 24

A 65-year-old man presents with intractable back pain. Radiographs reveal diffuse osteopenia and a vertebral compression fracture, while a skull radiograph shows punched-out lytic lesions.

What is the most definitive test to confirm the underlying diagnosis?

Explanation

While SPEP and urine immunofixation are standard screening tests, a bone marrow biopsy demonstrating greater than 10% clonal plasma cells is required for the definitive diagnosis of multiple myeloma. The presence of end-organ damage (CRAB criteria) further solidifies the clinical diagnosis.

Question 25

A 10-year-old child presents with bilateral leg pain and a waddling gait. Radiographs show symmetric, severe cortical thickening of the diaphyseal regions of the long bones.

What is the genetic mutation associated with this skeletal dysplasia?

Explanation

This presentation is classic for Progressive Diaphyseal Dysplasia (Camurati-Engelmann disease). It is an autosomal dominant condition caused by mutations in the TGFB1 gene, leading to increased bone formation and characteristic cortical thickening.

Question 26

A 45-year-old female is treated conservatively for a distal radius fracture. Eight weeks later, she develops severe burning pain out of proportion to the injury, along with skin changes and stiffness. Radiographs show patchy, periarticular osteopenia.

What is the best pharmacologic prophylaxis that could have been prescribed at the time of injury to reduce her risk of developing this condition?

Explanation

The patient has Complex Regional Pain Syndrome (CRPS) Type I. Vitamin C (500mg daily for 50 days) has been shown in some studies to reduce the incidence of CRPS following distal radius fractures.

Question 27

A 62-year-old patient with known multiple myeloma presents with a symptomatic lytic lesion in the proximal femur. The lesion involves 60% of the cortex, and the Mirels score is calculated to be 10. What is the most appropriate next step in management?

Explanation

A Mirels score of 9 or greater indicates a high risk for pathologic fracture (often >33%). Prophylactic surgical fixation is indicated, followed by radiation therapy to achieve local tumor control.

Question 28

Progressive Diaphyseal Dysplasia (Camurati-Engelmann disease) is a rare sclerosing bone dysplasia. Which of the following is the most common clinical presentation in childhood?

Explanation

Camurati-Engelmann disease classically presents with bone pain, delayed walking, proximal muscle weakness, and a waddling gait. Recurrent fractures are characteristic of Osteogenesis Imperfecta, not Camurati-Engelmann.

Question 29

A 35-year-old male develops chronic, severe burning pain and allodynia in his forearm following a recognized median nerve injury from a glass laceration. What feature distinguishes his diagnosis of CRPS Type II from CRPS Type I?

Explanation

CRPS is divided into Type I (formerly Reflex Sympathetic Dystrophy) and Type II (formerly Causalgia). The defining difference is that Type II involves a verified, definable peripheral nerve injury.

Question 30

A 30-year-old female presents with distal femur pain. Imaging reveals an eccentric, lytic, epiphyseal-metaphyseal lesion extending to the subchondral bone. Biopsy confirms Giant Cell Tumor (GCT). If medical therapy is considered, what is the mechanism of action of Denosumab?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL. In GCT, the neoplastic mononuclear cells express RANKL, which recruits reactive osteoclast-like giant cells; Denosumab halts this recruitment and subsequent bone destruction.

Question 31

A 4-year-old child with achondroplasia presents with progressively worsening lower extremity weakness, apnea, and hyperreflexia. What is the most likely anatomic cause of these symptoms?

Explanation

Infants and young children with achondroplasia are at high risk for cervicomedullary compression due to foramen magnum stenosis, leading to hyperreflexia, weakness, and central apnea. Lumbar stenosis typically presents later in adulthood.

Question 32

In a patient with advanced rheumatoid arthritis evaluating the cervical spine, which radiographic parameter indicates the highest risk for progressive neurologic deficit and dictates the need for surgical stabilization?

Explanation

The PADI (space available for the cord) is the most reliable predictor of neurologic recovery and risk. A PADI of less than 14 mm is a strong indication for surgical intervention in rheumatoid atlantoaxial subluxation.

Question 33

A 16-year-old male is diagnosed with a conventional high-grade osteosarcoma of the distal femur. He undergoes 10 weeks of neoadjuvant chemotherapy followed by wide surgical resection. What is the most significant prognostic factor for long-term survival in this patient?

Explanation

The degree of tumor necrosis after neoadjuvant chemotherapy (greater than or equal to 90% is considered a good response) is the most critical prognostic indicator for overall survival in conventional osteosarcoma.

Question 34

A 12-year-old boy presents with a permeative lytic lesion in the femoral diaphysis and an 'onion skin' periosteal reaction. Cytogenetics reveal a t(11;22) translocation. Which of the following fusion genes confirms the diagnosis?

Explanation

The t(11;22) translocation is the hallmark of Ewing Sarcoma, resulting in the EWS-FLI1 fusion transcript. SYT-SSX is associated with Synovial Sarcoma, and FUS-CHOP with Myxoid Liposarcoma.

Question 35

A 55-year-old patient presents with localized bone pain and a single lytic lesion in the ilium without any other symptomatic sites.

What strict criteria must be met to diagnose a Solitary Bone Plasmacytoma rather than Multiple Myeloma?

Explanation

Solitary bone plasmacytoma is diagnosed when there is a single biopsy-proven bone lesion of clonal plasma cells, with a normal skeletal survey/MRI, no end-organ damage (CRAB), and a bone marrow biopsy showing <10% plasma cells.

Question 36

A 6-year-old girl with blue sclerae, normal stature, and a history of three long bone fractures is diagnosed with Osteogenesis Imperfecta (OI) Type I. Which of the following best describes the underlying collagen defect?

Explanation

OI Type I is the mildest and most common form. It is characterized by a quantitative defect (null allele) resulting in decreased production of structurally normal Type I collagen.

Question 37

A 55-year-old male presents with podagra. Joint fluid aspiration reveals negatively birefringent, needle-shaped crystals. For long-term chronic management, he is prescribed Allopurinol. What is its mechanism of action?

Explanation

Allopurinol is a xanthine oxidase inhibitor that decreases uric acid production, serving as the first-line urate-lowering therapy for chronic gout. Colchicine inhibits microtubules, and probenecid is a uricosuric agent.

Question 38

A 15-year-old boy presents with severe, dull aching pain in his proximal thigh that worsens at night and is dramatically relieved by ibuprofen. Imaging shows an 8mm intracortical radiolucent nidus with dense surrounding sclerosis. What is the standard of care for definitive, minimally invasive treatment?

Explanation

The clinical and radiographic presentation is classic for an Osteoid Osteoma. CT-guided Radiofrequency Ablation (RFA) is the treatment of choice, offering high success rates with minimal morbidity.

Question 39

A 40-year-old woman develops CRPS Type I of the hand and wrist.

She is completely intolerant of physical therapy due to extreme allodynia, and she has failed NSAIDs, gabapentin, and oral steroids. Which intervention is most appropriate next to facilitate active range of motion therapy?

Explanation

When oral medications fail to control symptoms enough to allow physical therapy in upper extremity CRPS, a sympathetic block (Stellate ganglion block) is indicated to provide temporary pain relief and facilitate functional rehabilitation.

Question 40

A 25-year-old male with Multiple Hereditary Exostoses (MHE) presents with a rapidly enlarging and increasingly painful osteochondroma on his proximal tibia. Which of the following is the most critical complication to rule out?

Explanation

While nerve compression or bursitis can cause pain, a rapidly enlarging osteochondroma after skeletal maturity is highly suspicious for malignant transformation into a secondary peripheral chondrosarcoma.

Question 41

During preoperative planning for a total hip arthroplasty in a patient with severe rheumatoid arthritis, the surgeon evaluates for acetabular protrusio. Which radiographic finding confirms this diagnosis on an AP pelvis radiograph?

Explanation

Acetabular protrusio (protrusio acetabuli) is defined radiographically when the medial border of the femoral head projects medial to the ilioischial line (Köhler's line) on an AP pelvis radiograph.

Question 42

A 70-year-old man presents with progressive hearing loss and increasing hat size. Radiographs demonstrate thickened cortices and coarsened trabeculae in the skull and pelvis. Paget's disease is diagnosed. The mixed phase of this disease is characterized by which of the following cellular activities?

Explanation

Paget's disease has three main phases: an initial lytic phase (intense osteoclastic activity), a mixed phase (simultaneous massive osteoclastic and osteoblastic activity), and a sclerotic phase (predominant osteoblastic activity with disorganized woven bone).

Question 43

The mnemonic 'CRAB' is used to recall the hallmark end-organ damage manifestations of Multiple Myeloma. Which of the following accurately represents the CRAB criteria?

Explanation

The CRAB criteria defining end-organ damage in symptomatic multiple myeloma stand for hyperCalcemia, Renal insufficiency, Anemia, and Bone lesions (lytic).

Question 44

A 65-year-old male presents with new-onset back pain, fatigue, and hypercalcemia. Radiographs show multiple punched-out lytic lesions in the axial skeleton, as seen in the provided image. Which laboratory finding is most specific for confirming the diagnosis of this pathology?

Explanation

The clinical picture and lytic lesions represent multiple myeloma. A monoclonal spike (M-protein) on serum protein electrophoresis (SPEP) or urine protein electrophoresis (UPEP) is a hallmark finding, confirming the clonal plasma cell proliferation.

Question 45

A 12-year-old boy presents with severe leg pain, waddling gait, and muscle weakness. Radiographs demonstrate bilateral cortical thickening of the femoral and tibial diaphyses. A mutation in which of the following genes is responsible for this condition?

Explanation

The image and clinical presentation are characteristic of progressive diaphyseal dysplasia (Camurati-Engelmann disease). This autosomal dominant condition is caused by a mutation in the TGFB1 gene, leading to increased bone formation in the diaphyses.

Question 46

A 45-year-old female presents 8 weeks after ORIF of a distal radius fracture with severe burning pain, swelling, and shiny skin. Radiographs show patchy osteopenia. Which prophylactic medication given post-operatively could have significantly reduced her risk of developing this condition?

Explanation

The patient has Complex Regional Pain Syndrome (CRPS), also known as Sudeck's atrophy. Prophylactic administration of Vitamin C (500 mg daily for 50 days) after a distal radius fracture has been shown to decrease the incidence of CRPS.

Question 47

In the pathophysiology of multiple myeloma bone disease, which cytokine pathway is primarily responsible for the uncoupled osteoclast activation leading to the characteristic lytic lesions?

Explanation

Myeloma cells overexpress RANKL and reduce Osteoprotegerin (OPG) secretion in the bone marrow microenvironment. This severe imbalance directly drives massive osteoclast activation and the resulting lytic bone destruction.

Question 48

According to the Budapest criteria for the clinical diagnosis of Complex Regional Pain Syndrome (CRPS), which of the following symptom categories is NOT part of the diagnostic requirement?

Explanation

The Budapest criteria require at least one symptom in 3 of 4 categories: Sensory, Vasomotor, Sudomotor/edema, and Motor/trophic. Dermatomal sensory loss is indicative of a peripheral nerve injury or radiculopathy, not CRPS.

Question 49

What is the first-line medical treatment for managing severe bone pain and improving clinical symptoms in patients with Camurati-Engelmann disease (progressive diaphyseal dysplasia)?

Explanation

Systemic corticosteroids are the mainstay of medical treatment for progressive diaphyseal dysplasia. They have been shown to significantly reduce bone pain, improve fatigue, and sometimes reverse radiographic changes.

Question 50

A 60-year-old male with multiple myeloma presents with severe thigh pain on weight-bearing. Radiographs show a 4 cm lytic lesion in the subtrochanteric region with 50% cortical destruction. What is the most appropriate management?

Explanation

This lesion represents an impending pathologic fracture based on Mirels' criteria (size >2.5 cm, weight-bearing bone, pain, lytic nature). Prophylactic stabilization with an intramedullary nail followed by local radiation is the standard of care.

Question 51

A 4-year-old boy with achondroplasia presents with progressively worsening lower extremity weakness, central sleep apnea, and hyperreflexia. Evaluation of which anatomical region is most critical in this scenario?

Explanation

In infants and young children with achondroplasia, foramen magnum stenosis causing cervicomedullary compression is a life-threatening complication. It presents with hyperreflexia, weakness, hypotonia, and central sleep apnea, requiring urgent decompression.

Question 52

A 32-year-old female presents with a purely lytic, eccentric lesion in the distal femoral epiphysis. Biopsy reveals multinucleated giant cells in a background of mononuclear cells. The true neoplastic cells in this tumor express high levels of which marker?

Explanation

In a Giant Cell Tumor of bone, the mononuclear stromal cells are the true neoplastic elements, and they overexpress RANKL. This recruits and activates reactive multinucleated giant cells that express RANK, leading to massive bone resorption.

Question 53

A 55-year-old female with long-standing rheumatoid arthritis is scheduled for a total knee arthroplasty. Flexion-extension cervical spine radiographs reveal an anterior atlantoaxial subluxation (AAS) of 9 mm and a posterior atlantodental interval (PADI) of 12 mm. What is the most appropriate next step?

Explanation

In rheumatoid cervical spine involvement, a PADI of <14 mm is a critical threshold indicating impending cord compression. An MRI is required to evaluate for spinal cord impingement or myelomalacia before proceeding with any other management.

Question 54

In the treatment of conventional high-grade osteosarcoma, which specific finding is the most important independent prognostic factor for long-term survival?

Explanation

A good histologic response to neoadjuvant chemotherapy, defined as >90% tumor necrosis (Huvos Grade III or IV), is the most reliable predictor of overall and disease-free survival in patients with conventional osteosarcoma.

Question 55

A 14-year-old boy presents with a permeative lytic lesion in the femoral diaphysis with an "onion skin" periosteal reaction. A biopsy shows small round blue cells. Molecular analysis is most likely to reveal which of the following translocations?

Explanation

The clinical and radiographic presentation is classic for Ewing sarcoma. Over 85% of Ewing sarcomas are characterized by the t(11;22)(q24;q12) translocation, which creates the EWS-FLI1 fusion oncogene.

Question 56

In the evaluation of medial compartment osteoarthritis of the knee, which dynamic gait parameter is most closely correlated with disease severity and progression?

Explanation

The external knee adduction moment (KAM) during the stance phase of gait is an established surrogate measure of dynamic medial compartment loading. High KAM is strongly associated with the progression of medial knee osteoarthritis.

Question 57

A child is diagnosed with Osteogenesis Imperfecta Type I, which presents with mild bone fragility and blue sclerae. Which of the following best describes the underlying collagen defect in this specific type?

Explanation

Osteogenesis Imperfecta Type I is caused by a quantitative defect (a null allele), resulting in a decreased amount of structurally normal Type I collagen. In contrast, Types II, III, and IV involve qualitative (structural) defects, leading to more severe phenotypes.

Question 58

A 65-year-old male presents with acute severe pain and swelling in his knee. Joint aspiration reveals cloudy fluid. Polarized light microscopy shows weakly positively birefringent rhomboid-shaped crystals. This arthropathy is most strongly associated with which underlying condition?

Explanation

The crystals described are Calcium Pyrophosphate Dihydrate (CPPD), diagnostic of pseudogout. CPPD disease is highly associated with metabolic conditions, classically referred to as the "4 H's": Hemochromatosis, Hyperparathyroidism, Hypothyroidism, and Hypomagnesemia.

Question 59

A 55-year-old female sustains a distal radius fracture. To prevent the complication shown in the radiograph

, what pharmacological prophylaxis has historically been recommended during the conservative management of this injury?

Explanation

Vitamin C (500 mg daily for 50 days) has been shown in some studies to reduce the incidence of Complex Regional Pain Syndrome (CRPS) following distal radius fractures. CRPS is characterized by pain out of proportion, sudomotor changes, and patchy osteopenia.

Question 60

A 65-year-old male presents with severe mechanical back pain. A skeletal survey demonstrates multiple punched-out lytic lesions in the skull and spine.

Which of the following molecules, secreted by the malignant cells in this condition, is primarily responsible for the uncoupling of bone remodeling by directly inhibiting osteoblast differentiation?

Explanation

Multiple myeloma cells secrete Dickkopf-1 (DKK-1), which binds to LRP5/6 and inhibits the Wnt/beta-catenin signaling pathway. This prevents osteoblast differentiation, leading to purely lytic bone lesions without a blastic response.

Question 61

A 48-year-old female presents 12 weeks after a closed reduction of a distal radius fracture complaining of severe, burning hand pain. Examination reveals glossy, erythematous skin, profound stiffness, and hypersensitivity.

Which of the following criteria is the internationally accepted standard for diagnosing this condition?

Explanation

The Budapest criteria are the standardized clinical diagnostic criteria for Complex Regional Pain Syndrome (CRPS). They require the presence of disproportionate pain along with specific sensory, vasomotor, sudomotor/edema, and motor/trophic findings.

Question 62

A 10-year-old boy presents with progressive, deep-aching leg pain and a waddling gait. Radiographs reveal symmetrical, bilateral cortical thickening of the long bone diaphyses, sparing the metaphyses and epiphyses.

The pathogenesis of this disease is linked to a mutation in which of the following genes?

Explanation

Progressive diaphyseal dysplasia (Camurati-Engelmann disease) is caused by a mutation in the TGFB1 gene, which encodes transforming growth factor-beta 1. This leads to increased bone formation primarily along the diaphyseal cortices.

Question 63

A 6-month-old infant is evaluated for short limbs and a prominent forehead. Genetic testing confirms an activating mutation in the FGFR3 gene. At the cellular level within the physis, this mutation primarily results in which of the following?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3. Paradoxically, this receptor normally acts as a negative regulator of bone growth; its overactivation profoundly inhibits chondrocyte proliferation in the proliferative zone of the physis.

Question 64

A 28-year-old female presents with knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the distal femur. Biopsy reveals multinucleated giant cells interspersed among mononuclear spindle cells. She is prescribed denosumab. What is the precise cellular target and mechanism of this medication in her condition?

Explanation

In Giant Cell Tumor of bone, the true neoplastic cells are the mononuclear stromal cells, which express high levels of RANKL. Denosumab binds to this RANKL, preventing it from recruiting and activating the reactive, osteoclast-like giant cells.

Question 65

An 11-year-old boy presents with a painful, swollen mid-thigh. Radiographs demonstrate a destructive diaphyseal lesion with an 'onion skin' periosteal reaction. Histology shows sheets of small, round, blue cells. Which chromosomal translocation is most classically associated with this malignancy?

Explanation

Ewing sarcoma is classically characterized by the t(11;22)(q24;q12) translocation. This fuses the EWS gene on chromosome 22 with the FLI1 gene on chromosome 11, creating an oncogenic transcription factor.

Question 66

A 16-year-old girl with distal femur osteosarcoma undergoes 10 weeks of neoadjuvant chemotherapy, followed by a wide surgical resection. Pathological analysis of the resected specimen is performed. Which of the following is the most important prognostic factor for her long-term survival?

Explanation

The degree of tumor necrosis in response to neoadjuvant chemotherapy is the single most important prognostic factor in osteosarcoma. A 'good response' is defined as >90% tumor necrosis (Huvos Grade III or IV) and correlates with significantly higher survival rates.

Question 67

A 60-year-old woman with long-standing rheumatoid arthritis presents with neck pain, bilateral hand clumsiness, and hyperreflexia. Lateral flexion-extension radiographs reveal anterior atlantoaxial subluxation. Which of the following radiographic parameters is the most reliable predictor of neurologic deficit and the potential for recovery after surgery?

Explanation

In rheumatoid cervical myelopathy, the Posterior Atlantodental Interval (PADI), also known as the Space Available for the Cord (SAC), is the most critical parameter. A PADI < 14 mm strongly correlates with neurologic deficits and a lower chance of neurological recovery.

Question 68

A 62-year-old man presents with an impending pathologic fracture of the proximal femur secondary to a biopsy-proven metastatic renal cell carcinoma lesion. Prophylactic intramedullary nailing is planned. What is the most critical step in management immediately prior to surgical fixation?

Explanation

Metastatic lesions from renal cell carcinoma and thyroid cancer are notoriously hypervascular. Preoperative selective arterial embolization is critical to prevent catastrophic intraoperative hemorrhage during orthopedic stabilization.

Question 69

A 22-year-old female undergoes corrective osteotomy for a 'shepherd's crook' deformity of the proximal femur. Pathology demonstrates a stroma of bland spindle cells with irregularly shaped trabeculae of woven bone lacking osteoblastic rimming. This condition is caused by a somatic activating mutation in which gene?

Explanation

The clinical and histologic description ('Chinese character' woven bone) is classic for Fibrous Dysplasia. This disease is caused by a somatic missense mutation in the GNAS gene, which encodes the stimulatory G-protein alpha subunit, leading to elevated cAMP levels.

Question 70

A 32-year-old man presents with recurrent, spontaneous hemarthroses of the knee. MRI demonstrates a nodular synovial mass with prominent 'blooming artifact' on gradient-echo sequences. The pathophysiology of this condition involves the overexpression of which of the following factors?

Explanation

Tenosynovial Giant Cell Tumor (formerly Pigmented Villonodular Synovitis) demonstrates 'blooming artifact' due to hemosiderin. It is driven by a t(1;2) translocation that causes overexpression of CSF1, which recruits inflammatory macrophages that form the bulk of the tumor mass.

Question 71

A 55-year-old man presents with acute, excruciating pain and swelling in his first metatarsophalangeal joint. Joint aspiration yields cloudy fluid. Under polarized light microscopy, which of the following findings confirms the diagnosis of gout?

Explanation

Gout is characterized by the deposition of monosodium urate crystals, which appear needle-shaped and exhibit strong negative birefringence (yellow when parallel to the axis of the compensator) under polarized light microscopy.

Question 72

A 12-year-old boy with a family history of bone bumps presents with multiple cartilage-capped bony outgrowths around his knees and wrists. He is diagnosed with Multiple Hereditary Exostoses. The genetic mutations responsible for this condition disrupt normal physeal growth by impairing which cellular process?

Explanation

Multiple Hereditary Exostoses (MHE) is caused by mutations in the EXT1 or EXT2 genes. These genes act in the Golgi apparatus to synthesize heparan sulfate. Deficient heparan sulfate disrupts the diffusion of Indian hedgehog (Ihh) signaling at the growth plate, leading to exostosis formation.

Question 73

A newborn is noted to have a large anterior fontanelle, delayed closure of cranial sutures, and hypermobility of the shoulders, allowing them to be approximated in the anterior midline. Radiographs reveal hypoplastic clavicles. This condition is caused by a mutation in a gene essential for the differentiation of which cell type?

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (also known as CBFA1), a master transcription factor essential for osteoblast differentiation and intramembranous ossification (affecting the clavicles and skull).

Question 74

A 4-year-old child presents with short-limbed dwarfism, frontal bossing, and midface hypoplasia. Radiographs show narrowing of the interpedicular distances in the lumbar spine. Which genetic mutation and receptor anomaly are responsible for this condition?

Explanation

Achondroplasia is caused by a gain-of-function (activating) mutation in the FGFR3 gene. This leads to the inhibition of chondrocyte proliferation in the proliferative zone of the physis.

Question 75

A 70-year-old man presents with severe back pain and anemia. Radiographs demonstrate multiple lytic skull lesions.

Serum electrophoresis shows an M-spike. Which of the following is the most likely finding on bone marrow biopsy?

Explanation

The classic presentation and radiographic "punched-out" lesions denote multiple myeloma. The diagnosis is confirmed by finding greater than 10% clonal plasma cells on a bone marrow biopsy.

Question 76

A 55-year-old male undergoes arthrocentesis for a swollen, painful knee. Synovial fluid analysis reveals negatively birefringent, needle-shaped crystals under polarized light microscopy. What is the composition of these crystals?

Explanation

Gout is characterized by monosodium urate crystals that are needle-shaped and strongly negatively birefringent under polarized light. Calcium pyrophosphate crystals, seen in pseudogout, are rhomboid and weakly positively birefringent.

Question 77

A 32-year-old female presents with knee pain. Radiographs show an eccentric, purely lytic lesion in the distal femur epiphysis extending to the subchondral bone. A biopsy reveals multinucleated giant cells. Which of the following targeted therapies is most appropriate for unresectable disease?

Explanation

Giant Cell Tumor of bone consists of neoplastic stromal cells that express RANKL, which recruits osteoclast-like giant cells. Denosumab, a RANKL monoclonal antibody, is highly effective for unresectable or recurrent GCT.

Question 78

A 6-year-old boy has sustained multiple low-energy fractures. Examination shows blue sclerae and mild hearing loss. The primary underlying defect in this condition involves which of the following?

Explanation

Osteogenesis Imperfecta is most commonly caused by mutations in COL1A1 or COL1A2. This results in defective synthesis of Type I collagen, affecting primarily bone, sclera, and hearing.

Question 79

A 14-year-old boy has a permeative, destructive lesion in the distal femur with a "sunburst" periosteal reaction. Biopsy confirms high-grade intramedullary osteosarcoma. What is the single most important prognostic factor for his long-term survival?

Explanation

The degree of tumor necrosis following neoadjuvant chemotherapy is the most critical prognostic factor for long-term survival in high-grade osteosarcoma. Greater than 90% necrosis indicates a favorable response.

Question 80

A 50-year-old female presents 3 months after a conservatively treated distal radius fracture with a stiff, swollen, erythematous hand, and severe allodynia. Radiographs show periarticular osteopenia.

What is the primary autonomic nervous system dysfunction mediating her symptoms?

Explanation

Complex Regional Pain Syndrome (CRPS) is driven by abnormal sympathetic nervous system hyperactivity. This leads to vasomotor and sudomotor changes, severe allodynia, and trophic skin changes.

Question 81

A 60-year-old woman with long-standing rheumatoid arthritis presents with new-onset myelopathy. Flexion-extension cervical radiographs reveal an atlantodental interval (ADI) of 10 mm. Which ligament's failure is primarily responsible for this radiographic finding?

Explanation

Atlantoaxial subluxation in rheumatoid arthritis is primarily due to the destruction and laxity of the transverse ligament. This is caused by the invasive synovial pannus at the atlanto-odontoid joint.

Question 82

A 12-year-old boy presents with thigh pain and fever. Radiographs show a diaphyseal lytic lesion of the femur with an "onion-skin" periosteal reaction. Biopsy shows small blue round cells. Which chromosomal translocation is most characteristically associated with this tumor?

Explanation

Ewing sarcoma is classically characterized by the t(11;22)(q24;q12) translocation. This specific genetic alteration results in the formation of the EWS-FLI1 fusion protein.

Question 83

A 10-year-old girl exhibits hypermobility of her shoulders allowing them to meet in the midline. She also has delayed eruption of permanent teeth. What is the most likely genetic mutation responsible for her condition?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by mutations in the CBFA1 (RUNX2) gene, essential for osteoblast differentiation. It classically presents with hypoplastic clavicles and delayed skull ossification.

Question 84

A 65-year-old man presents with chronic hip pain. Radiographs show a large destructive lesion in the ilium with "rings and arcs" calcification. Biopsy confirms a grade II conventional chondrosarcoma. What is the most appropriate primary treatment?

Explanation

Conventional chondrosarcomas are largely resistant to both chemotherapy and radiation therapy. Wide surgical resection with negative margins is the mainstay of treatment for intermediate and high-grade lesions.

Question 85

A 65-year-old man presents with increasing head size and dull, aching pain in his right thigh. Radiographs of the femur demonstrate a thickened cortex with a "blade of grass" lucency at the leading edge. What is the primary cellular defect in this condition?

Explanation

Paget disease of bone is initiated by overactive, large, multinucleated osteoclasts that heavily resorb bone. This is followed by a disorganized, excessive osteoblastic bone formation phase.

Question 86

In the pathophysiology of primary osteoarthritis, progressive articular cartilage degradation is primarily driven by upregulation of which of the following enzymes by chondrocytes?

Explanation

Matrix metalloproteinases (especially MMP-1, MMP-3, and MMP-13), produced by stressed chondrocytes, are the primary enzymes responsible for the degradation of type II collagen and aggrecan in osteoarthritis.

Question 87

A 45-year-old woman undergoes an MRI for knee pain, which reveals a 4 cm cartilage tumor in the distal femur. Which of the following features is the most reliable indicator of a grade I chondrosarcoma rather than a benign enchondroma?

Explanation

Deep endosteal scalloping (>2/3 of the cortical thickness), cortical breakthrough, and soft tissue extension are key radiographic signs. These findings help differentiate a low-grade chondrosarcoma from a benign enchondroma.

Question 88

A 16-year-old boy presents with a 6-month history of right thigh pain that is worse at night and completely relieved by ibuprofen. Radiographs show a 1 cm radiolucent nidus surrounded by dense sclerotic bone. The profound pain relief from NSAIDs is due to inhibition of which substance produced by the nidus?

Explanation

Osteoid osteomas classically cause night pain relieved by NSAIDs. This occurs because the nidus produces extremely high levels of Prostaglandin E2 (PGE2), which is inhibited by cyclooxygenase (COX) inhibitors.

Question 89

A 15-year-old male presents with enlarging bumps around his knees and ankles. Radiographs demonstrate multiple sessile and pedunculated osteochondromas. He carries a mutation in the EXT1 gene. What is the approximate lifetime risk of malignant transformation to chondrosarcoma in this condition?

Explanation

Multiple Hereditary Exostoses (MHE) is caused by mutations in the EXT1 or EXT2 genes. The lifetime risk of an osteochondroma undergoing malignant transformation to secondary chondrosarcoma in MHE is approximately 1-5%.

Question 90

A newborn presents with short-limbed dwarfism, "hitchhiker" thumbs, clubfeet, and "cauliflower" ears. Radiographs reveal severely shortened first metacarpals. What is the underlying genetic defect in this dysplasia?

Explanation

Diastrophic dysplasia is an autosomal recessive disorder caused by a mutation in the SLC26A2 (DTDST) gene. This leads to defective sulfate transport and undersulfated proteoglycans in cartilage.

Question 91

A 68-year-old male presents with fatigue, diffuse bone pain, and hypercalcemia. A representative radiograph is shown.

Serum protein electrophoresis confirms a monoclonal gammopathy. When evaluating the extent of this patient's skeletal disease, which of the following imaging modalities is most likely to yield false-negative results due to the underlying pathophysiology of the bone lesions?

Explanation

Technetium-99m bone scintigraphy relies on osteoblastic activity for radiotracer uptake. In multiple myeloma, neoplastic cells secrete factors (such as DKK-1) that profoundly suppress osteoblasts, leading to purely lytic lesions that frequently do not demonstrate uptake on traditional bone scans.

Skeletal Dysplasias of the Spine - Arab Board MCQ Prep

ABOS Board Review: Orthopedic Pathology, Bone Tumors, Skeletal Dysplasias, Arthritis | Part 12

Key Takeaway

ABOS Board Review: Orthopedic Pathology, Bone Tumors, Skeletal Dysplasias, Arthritis | Part 12

Chapter Index