Question 1

Which of the following correctly describes the underlying molecular pathophysiology in Neurofibromatosis Type 1 (NF1)?

Explanation

NF1 is caused by a mutation in the NF1 gene on chromosome 17, which encodes the protein neurofibromin. Neurofibromin normally acts as a tumor suppressor by downregulating the Ras-MAPK pathway; its loss leads to unchecked Ras upregulation and cellular proliferation.

Question 2

A 6-year-old child with NF1 presents with established anterolateral bowing and non-union of the tibia.

What is the primary histological finding at the site of the pseudoarthrosis that impairs healing?

Explanation

In congenital anterolateral bowing of the tibia associated with NF1, the pseudoarthrosis site is enveloped by a thickened, fibrous, hamartomatous periosteum. This abnormal tissue acts as a mechanical and biological barrier, severely impairing local vascularization and bone union.

Question 3

A 14-year-old male is diagnosed with high-grade conventional osteosarcoma of the distal femur. What is the standard first-line neoadjuvant chemotherapy regimen?

Explanation

The standard first-line neoadjuvant chemotherapy regimen for conventional osteosarcoma is the MAP regimen. This includes Methotrexate (high-dose), Adriamycin (Doxorubicin), and Platinol (Cisplatin).

Question 4

Which radiographic feature in a patient with NF1 is most predictive of rapid progression of dystrophic scoliosis?

Explanation

Dystrophic scoliosis in NF1 is characterized by short, sharp curves, rib penciling, severe vertebral scalloping, and spindling of the transverse processes. These dystrophic features indicate a high risk for rapid curve progression, often necessitating early surgical intervention.

Question 5

What is the primary function of the protein mutated in Neurofibromatosis type 1?

Explanation

NF1 is caused by a mutation in the NF1 gene on chromosome 17q11.2, which encodes neurofibromin. Neurofibromin acts as a tumor suppressor by downregulating the Ras signaling pathway via its GTPase-activating protein (GAP) activity.

Question 6

A 10-year-old boy with NF1 presents with a short, sharply angulated thoracic scoliosis. Radiographs show rib penciling and severe vertebral scalloping. What is the most appropriate surgical management?

Explanation

Dystrophic scoliosis in NF1 features a short, sharp curve, rib penciling, and vertebral scalloping. Due to the high risk of pseudoarthrosis and curve progression, combined anterior and posterior spinal fusion is the standard of care.

Question 7

A 2-year-old child with NF1 presents with anterolateral bowing of the tibia and an impending fracture as seen in the clinical and radiographic image.

What is the gold standard surgical principle for managing established tibial pseudoarthrosis in this condition?

Explanation

Congenital pseudoarthrosis of the tibia in NF1 requires aggressive management. Successful treatment involves complete resection of the hamartomatous tissue, robust intramedullary fixation, and extensive bone grafting.

Question 8

A 16-year-old male with a history of bilateral retinoblastoma develops a conventional osteosarcoma of the distal femur. This patient is most likely to harbor a germline mutation in which of the following genes?

Explanation

Germline mutations in the RB1 gene are responsible for hereditary retinoblastoma and significantly increase the risk of secondary malignancies, particularly osteosarcoma. TP53 mutations are associated with Li-Fraumeni syndrome, which also predisposes to osteosarcoma.

Question 9

A 45-year-old female presents with persistent, unprovoked deep shoulder pain. Radiographs reveal a stippled calcific lesion in the proximal humerus with endosteal scalloping affecting 80% of the cortical thickness. What is the most likely diagnosis?

Explanation

Endosteal scalloping greater than two-thirds of the cortical thickness and persistent, unprovoked pain are highly suspicious for malignant transformation of an enchondroma into a secondary chondrosarcoma.

Question 10

Which chromosomal translocation is the diagnostic hallmark of Ewing sarcoma?

Explanation

Ewing sarcoma is classically associated with the t(11;22) chromosomal translocation, resulting in the EWS-FLI1 fusion protein. By contrast, t(X;18) is characteristic of synovial sarcoma.

Question 11

A 30-year-old female presents with an expansile, lytic lesion in the distal femur extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor. What is the mechanism of action of Denosumab, a frequently used neoadjuvant therapy?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This inhibits osteoclast-mediated bone destruction, which is driven by the neoplastic mononuclear cells in GCT.

Question 12

A 35-year-old male with NF1 presents with a rapidly enlarging, painful mass in his left thigh that originated from a long-standing plexiform neurofibroma. What is the most likely diagnosis?

Explanation

Patients with NF1 have a 5-10% lifetime risk of developing a Malignant Peripheral Nerve Sheath Tumor (MPNST). These aggressive sarcomas typically arise from pre-existing plexiform neurofibromas and present with rapid growth and severe pain.

Question 13

Multiple Hereditary Exostoses (MHE) is inherited in an autosomal dominant pattern. Mutations in EXT1 and EXT2 genes disrupt the synthesis of which of the following?

Explanation

EXT1 and EXT2 mutations in MHE lead to a defect in heparan sulfate synthesis. This disruption causes abnormal chondrocyte proliferation and the formation of multiple osteochondromas.

Question 14

A 12-year-old girl presents with a "shepherd's crook" deformity of the proximal femur, precocious puberty, and large café-au-lait spots with irregular borders. This syndrome is caused by a somatic mutation affecting which signaling pathway?

Explanation

McCune-Albright syndrome consists of polyostotic fibrous dysplasia, endocrine abnormalities, and "coast of Maine" café-au-lait spots. It is caused by a post-zygotic somatic activating mutation in the GNAS gene, which upregulates the cAMP signaling pathway.

Question 15

A 22-year-old male presents with chronic back pain that is poorly relieved by NSAIDs. Imaging reveals a 2.5 cm lytic lesion with central ossification in the posterior elements of L4. What is the most likely diagnosis?

Explanation

Osteoblastomas are histologically similar to osteoid osteomas but are larger (>1.5 to 2.0 cm). They frequently occur in the posterior elements of the spine and present with dull pain that is characteristically less responsive to NSAIDs.

Question 16

A 65-year-old male presents with generalized bone pain and a pathological fracture of the proximal humerus. Radiographs show multiple punched-out lytic lesions, but a technetium-99m bone scan is negative. What is the most appropriate initial diagnostic laboratory test?

Explanation

Multiple myeloma classically presents with punched-out lytic lesions that do not elicit an osteoblastic response, resulting in a "cold" bone scan. Serum and urine protein electrophoresis (SPEP/UPEP) are the best initial tests to detect a monoclonal paraprotein spike.

Question 17

A 55-year-old male with a history of renal cell carcinoma presents with a painful, impending pathological fracture of the proximal femur due to a large lytic metastasis. What is the most critical pre-operative intervention prior to prophylactic stabilization?

Explanation

Renal cell carcinoma and thyroid carcinoma metastases are highly vascular and prone to massive intraoperative bleeding. Pre-operative embolization within 24 to 48 hours of surgery is critical to minimize blood loss during stabilization.

Question 18

A 28-year-old male presents with a slow-growing, painful mass near his knee joint. Radiographs reveal a soft tissue mass with stippled calcifications. Biopsy demonstrates a biphasic pattern of spindle and epithelial cells. What is the characteristic genetic mutation?

Explanation

Synovial sarcoma classically presents in young adults as a calcified soft tissue mass near a joint. It is characterized by the t(X;18) translocation, resulting in the SYT-SSX fusion gene.

Question 19

The gene mutated in Neurofibromatosis Type 1 (NF1) encodes for the protein neurofibromin. What is the normal cellular function of this protein?

Explanation

Neurofibromin functions as a GTPase-activating protein (GAP) that normally inhibits the Ras signaling pathway. Loss of neurofibromin leads to constitutively active Ras, driving uncontrolled cell proliferation and tumor formation in NF1 patients.

Question 20

A 7-year-old child with Neurofibromatosis Type 1 (NF1) presents with a rapidly progressive, 55-degree thoracic scoliotic curve. Radiographs show severe apical vertebral scalloping, rib penciling, and a short, sharp curve pattern. What is the most appropriate definitive management?

Explanation

Dystrophic scoliosis in NF1 is characterized by a short, sharp curve with severe bony changes like vertebral scalloping and rib penciling. Due to the high risk of rapid progression and pseudarthrosis, the gold standard treatment for severe dystrophic curves is a combined anterior and posterior spinal fusion.

Question 21

An 8-month-old infant with multiple café-au-lait spots is noted to have isolated anterolateral bowing of the tibia. Radiographs confirm a dysplastic tibia without a fracture.

What is the recommended initial management to alter the natural history of this condition?

Explanation

Anterolateral bowing in NF1 signals impending congenital pseudarthrosis of the tibia (CPT). Before a fracture occurs, standard initial management is a total contact orthosis (bracing) to protect the tibia and prevent or delay fracture, although the natural history often eventually leads to fracture.

Question 22

During surgical resection of an established congenital pseudarthrosis of the tibia (CPT) in a patient with NF1, the thick, collar-like tissue surrounding the pseudarthrosis site is excised. What is the primary histological characteristic of this excised tissue?

Explanation

The tissue surrounding a CPT in NF1 patients is a fibrous hamartoma that behaves like a thickened, constrictive periosteum. It lacks normal osteogenic potential and must be widely excised during reconstructive surgery to allow proper bone healing.

Question 23

A 15-year-old boy presents with knee pain. Radiographs reveal an eccentric, lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor (GCT). Which cellular component is the true neoplastic cell, and which signaling molecule does it express to drive osteolysis?

Explanation

In GCT of bone, the spindle-shaped mononuclear stromal cells are the true neoplastic elements. They overexpress RANKL, which recruits and activates normal host macrophages to fuse into the osteolytic multinucleated giant cells.

Question 24

A 28-year-old female with a known history of Neurofibromatosis Type 1 (NF1) and multiple plexiform neurofibromas presents with rapid enlargement and severe pain in a pre-existing thigh mass. Which of the following imaging modalities is considered the most sensitive for detecting malignant transformation in this patient?

Explanation

FDG-PET/CT is highly sensitive for distinguishing benign plexiform neurofibromas from malignant peripheral nerve sheath tumors (MPNSTs) in NF1 patients. High Standardized Uptake Values (SUV) strongly suggest malignant transformation, guiding the biopsy site.

Question 25

A patient diagnosed with Ewing sarcoma undergoes cytogenetic testing. Which chromosomal translocation and resulting fusion gene is most classically associated with this malignancy?

Explanation

Ewing sarcoma is classically driven by the t(11;22)(q24;q12) translocation, which produces the EWS-FLI1 fusion protein. This occurs in approximately 85-90% of cases and serves as a vital diagnostic marker.

Question 26

Following neoadjuvant chemotherapy, a 16-year-old patient with conventional high-grade osteosarcoma undergoes wide surgical resection. The pathology report details the Huvos grading. What minimum percentage of tumor necrosis is required to be classified as a "good" histologic response?

Explanation

A "good" histologic response to neoadjuvant chemotherapy in osteosarcoma is defined as greater than 90% tumor necrosis (Huvos Grade III or IV). This is one of the most important prognostic factors for long-term survival.

Question 27

A 13-year-old girl presents with pain in her left shoulder. Radiographs demonstrate a purely lytic, well-circumscribed lesion strictly localized to the proximal humeral epiphysis. She has open physes. What is the most likely diagnosis?

Explanation

Chondroblastoma classically presents as a purely lytic lesion in the epiphysis or apophysis of a skeletally immature patient (open physes). Giant cell tumors also occur in the epiphysis but almost exclusively after physeal closure.

Question 28

Which of the following primary carcinomas is most notorious for producing highly vascular, purely lytic osseous metastases that are largely resistant to radiation therapy, making wide resection or stabilization the preferred management?

Explanation

Renal cell carcinoma (RCC) metastases to bone are characteristically hypervascular and radioresistant. Preoperative embolization is often mandatory to prevent massive hemorrhage during surgical stabilization or wide resection.

Question 29

A 9-year-old child with NF1 presents with a significant spinal deformity.

Reviewing the radiographs, all of the following are classic radiologic features of a dystrophic NF1 curve EXCEPT:

Explanation

Dystrophic curves in NF1 are typically short, sharp, and angular, rather than long and sweeping. Vertebral scalloping, rib penciling, and widened pedicles due to dural ectasia are classic hallmark features.

Question 30

A 24-year-old female presents with a painless, slow-growing mass on the posterior aspect of her distal femur. Radiographs reveal a heavily ossified, lobulated mass on the cortical surface with a radiolucent cleft separating the tumor from the underlying cortex. What is the most likely diagnosis?

Explanation

Parosteal osteosarcoma is a low-grade surface osteosarcoma classically found on the posterior distal femur. The radiolucent cleft between the heavily ossified tumor mass and the host bone cortex is known as the "string sign".

Question 31

A 45-year-old patient presents with six large café-au-lait spots, axillary freckling, and Lisch nodules. He states his mother had similar skin findings and a leg deformity. What is the inheritance pattern and the chromosome involved in this disorder?

Explanation

Neurofibromatosis Type 1 (NF1) is an autosomal dominant genetic disorder caused by a mutation in the NF1 gene located on Chromosome 17. NF2 involves Chromosome 22.

Question 32

A 5-year-old boy is diagnosed with Neurofibromatosis type 1 (NF-1). Which of the following best describes the molecular pathophysiology of his condition?

Explanation

NF-1 is caused by a mutation in the NF1 gene on chromosome 17, which encodes the tumor suppressor neurofibromin. Neurofibromin normally downregulates the Ras-MAPK pathway; its loss leads to uncontrolled cell proliferation and tumor formation.

Question 33

A 3-year-old girl with Neurofibromatosis type 1 presents with anterolateral bowing of her left tibia. Radiographs show narrowing of the medullary canal and early cortical thickening. What is the natural history of this specific tibial deformity if left untreated?

Explanation

Anterolateral bowing of the tibia in NF-1 is highly associated with congenital pseudarthrosis of the tibia (CPT). Due to abnormal, thick hamartomatous periosteum impairing osteogenesis, fractures in this area typically result in intractable nonunions.

Question 34

A 10-year-old boy with NF-1 presents with a 45-degree short, sharp thoracic scoliotic curve. Radiographs demonstrate rib penciling, vertebral scalloping, and severe rotation. What is the most appropriate definitive management?

Explanation

This patient has dystrophic scoliosis associated with NF-1, characterized by rapid progression, short sharp curves, rib penciling, and vertebral scalloping. Combined anterior and posterior spinal fusion is required because isolated posterior fusions have unacceptably high pseudarthrosis and curve progression rates.

Question 35

A 14-year-old boy undergoes neoadjuvant chemotherapy followed by wide resection for a conventional high-grade osteosarcoma of the distal femur. Which of the following factors provides the most important prognostic information for his long-term survival?

Explanation

The histologic response to neoadjuvant chemotherapy, measured by the percentage of tumor necrosis in the resected specimen, is the single most important prognostic indicator for overall survival in osteosarcoma. Greater than 90% necrosis is defined as a good response.

Question 36

A 12-year-old girl presents with a painful, swollen mid-thigh. Radiographs show a permeative diaphyseal lesion with an "onion-skin" periosteal reaction. A biopsy reveals sheets of uniform, small blue round cells. Which of the following genetic translocations is most characteristic of this diagnosis?

Explanation

Ewing sarcoma typically presents as a permeative diaphyseal lesion with an "onion-skin" periosteal reaction and consists of small round blue cells expressing CD99. The hallmark cytogenetic abnormality is the t(11;22) translocation, resulting in the EWS-FLI1 fusion protein.

Question 37

A 65-year-old man presents with deep, persistent pelvic pain. Radiographs reveal a large, ill-defined destructive lesion in the ilium with "ring and arc" calcifications. Core needle biopsy confirms a grade 2 conventional chondrosarcoma. What is the most appropriate primary treatment modality?

Explanation

Conventional high-grade and intermediate-grade (grade 2) chondrosarcomas are generally resistant to both chemotherapy and radiation therapy. The definitive treatment is wide surgical resection to achieve negative margins.

Question 38

A 35-year-old man with a known history of Neurofibromatosis type 1 reports that a long-standing, palpable mass in his thigh has recently doubled in size and become exquisitely painful. MRI shows an enlarging, heterogeneous mass along the sciatic nerve. What is the most likely diagnosis?

Explanation

Patients with NF-1 have an 8-10% lifetime risk of developing a Malignant Peripheral Nerve Sheath Tumor (MPNST). Sudden rapid growth and new-onset severe pain in a pre-existing plexiform neurofibroma strongly suggest malignant transformation.

Question 39

A 4-year-old child with a history of multiple low-energy fractures, blue sclerae, and dentinogenesis imperfecta is being evaluated. The underlying molecular defect in this condition primarily affects which of the following processes?

Explanation

Osteogenesis Imperfecta is typically caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes. This leads to the substitution of glycine by bulkier amino acids, disrupting the proper formation and folding of the type I collagen triple helix.

Question 40

A 28-year-old woman presents with knee pain. Radiographs show an eccentric, expansile, lytic lesion in the distal femur extending to the subchondral bone without a sclerotic margin. Biopsy shows multinucleated giant cells in a background of mononuclear stromal cells. If systemic medical therapy is indicated, which molecular pathway is the primary target?

Explanation

Giant cell tumors of bone are driven by neoplastic mononuclear stromal cells that overexpress RANKL, which aggressively recruits and activates osteoclast-like multinucleated giant cells. Denosumab, a monoclonal antibody against RANKL, blocks this pathway and is used for advanced or inoperable cases.

Question 41

A 5-year-old boy is diagnosed with Neurofibromatosis Type 1. Which of the following best describes the genetic pathophysiology of this condition?

Explanation

NF1 is an autosomal dominant condition caused by a mutation on chromosome 17. The mutation leads to reduced neurofibromin, an inhibitor of the Ras pathway, causing unregulated cell growth and tumor formation.

Question 42

A 14-month-old child with multiple café-au-lait macules presents with the following tibial deformity.

What is the initial orthotic management and primary natural history concern?

Explanation

Anterolateral bowing of the tibia is heavily associated with NF1 and carries a high risk of progressing to congenital pseudarthrosis of the tibia (CPT). Early management involves total contact bracing to protect against fracture.

Question 43

When performing an incisional biopsy of a suspected primary malignant bone tumor in the distal femur, which of the following is an absolute surgical principle?

Explanation

The biopsy tract is considered contaminated with tumor cells and must be excised en bloc during definitive surgery. Longitudinal incisions are used to facilitate future wide excision, and meticulous hemostasis (deflating the tourniquet prior to closure) prevents tumor-seeding hematomas.

Question 44

A 10-year-old girl with NF1 presents with a rapidly progressing spinal deformity. Radiographs demonstrate short-segment, sharp angular curves with vertebral scalloping and rib penciling.

What is the most appropriate surgical strategy?

Explanation

Dystrophic scoliosis in NF1 is characterized by severe, rapidly progressive, short-segment curves with dysplastic osseous features. Due to the high risk of pseudarthrosis and curve progression, a combined anterior and posterior spinal fusion is the recommended standard of care.

Question 45

A 16-year-old boy presents with knee pain. Radiographs reveal a metaphyseal permeative lesion in the distal femur with a 'sunburst' periosteal reaction. What is the most important prognostic factor for his long-term survival following neoadjuvant chemotherapy and wide resection?

Explanation

The percentage of tumor necrosis following neoadjuvant chemotherapy is the single most important prognostic factor for overall survival in osteosarcoma. Greater than 90% necrosis indicates a good response to chemotherapy and a significantly better prognosis.

Question 46

A 12-year-old boy presents with a diaphyseal femur mass, fever, and elevated ESR. Radiographs show an 'onion-skin' periosteal reaction. Cytogenetics reveal a t(11;22) translocation. Which fusion protein is characteristic of this tumor?

Explanation

Ewing sarcoma is classically characterized by the t(11;22) translocation resulting in the EWS-FLI1 fusion protein. This cytogenetic marker is present in approximately 85-90% of Ewing sarcoma cases.

Question 47

A 28-year-old female presents with knee pain. Radiographs show an eccentric, lytic epiphyseal lesion in the proximal tibia. Biopsy confirms Giant Cell Tumor (GCT). If medical management is considered prior to curettage to downstage the tumor, what is the mechanism of action of the preferred agent?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, inhibiting osteoclast differentiation and function. It is frequently used in challenging GCT cases to ossify the tumor periphery and facilitate a more complete curettage.

Question 48

A 14-year-old boy complains of right thigh pain that is worse at night and dramatically relieved by ibuprofen. CT scan shows a 7 mm radiolucent nidus surrounded by dense sclerosis in the femoral diaphysis. What is the primary mediator responsible for his specific pain pattern?

Explanation

Osteoid osteomas characteristically secrete high levels of Prostaglandin E2 (PGE2) from the nidus. This leads to intense, night-time pain that is highly responsive to NSAIDs.

Question 49

A 55-year-old man presents with an enlarging, painful mass in his proximal femur. Radiographs show a lytic lesion with 'popcorn' calcifications and endosteal scalloping. What is the standard of care for a conventional high-grade chondrosarcoma?

Explanation

Conventional chondrosarcomas are notoriously resistant to both chemotherapy and radiation therapy due to their poor vascularity and slow growth fraction. Wide surgical resection is the primary and most effective definitive treatment.

Question 50

A 9-year-old boy with Neurofibromatosis Type 1 (NF1) presents with a rapidly progressing thoracic scoliosis. Radiographs demonstrate a short, sharp curve with vertebral scalloping, severe apical wedging, and penciling of the ribs. Which of the following is the most appropriate surgical strategy for this specific type of curve?

Explanation

Dystrophic scoliosis in NF1 is characterized by short, sharp curves, vertebral scalloping, and rib penciling. Due to a high risk of pseudoarthrosis and rapid curve progression, combined anterior and posterior spinal fusion is the recommended surgical treatment.

Question 51

A 5-year-old child with NF1 presents with anterolateral bowing of the tibia and a frank pseudoarthrosis.

What is the gold standard surgical management to achieve union and maintain alignment in this condition?

Explanation

The gold standard for treating congenital pseudoarthrosis of the tibia in NF1 involves complete resection of the fibrous hamartoma, intramedullary fixation to control alignment and guide growth, and robust autogenous bone grafting.

Question 52

A 32-year-old female presents with a destructive, expansile, purely lytic lesion in the distal femur epiphysis. Biopsy confirms a Giant Cell Tumor (GCT) of bone. She is treated preoperatively with denosumab. What is the primary mechanism of action of this medication in the context of GCT?

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, which is overexpressed by the neoplastic mononuclear stromal cells in GCT. This prevents RANKL from binding to RANK on osteoclast precursors, thereby halting the formation of bone-resorbing giant cells.

Question 53

A 12-year-old boy presents with thigh pain and fever. Radiographs show a permeative diaphyseal lesion with an 'onion skin' periosteal reaction. A core needle biopsy is performed. Which of the following cytogenetic abnormalities is most characteristic of this diagnosis?

Explanation

The patient has Ewing sarcoma, classically presenting as a permeative diaphyseal lesion with onion-skin periostitis. The characteristic translocation is t(11;22)(q24;q12), which results in the oncogenic EWS-FLI1 fusion protein.

Question 54

A 35-year-old male with a known diagnosis of Neurofibromatosis Type 1 reports rapid enlargement and new onset of severe, rest pain in a pre-existing plexiform neurofibroma in his thigh.

What is the most appropriate next step to evaluate for malignant transformation?

Explanation

Rapid growth and new rest pain in a plexiform neurofibroma strongly suggest transformation to a Malignant Peripheral Nerve Sheath Tumor (MPNST). An FDG-PET scan is highly sensitive for identifying malignant areas (high SUV) to guide an accurate biopsy.

Question 55

A 55-year-old male presents with shoulder pain. Radiographs reveal a large, destructive lesion in the proximal humerus with intralesional 'popcorn' calcifications. Biopsy confirms a grade 2 conventional chondrosarcoma. What is the most appropriate definitive management?

Explanation

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

Question 56

A 22-year-old male presents with dull, aching pain in his mid-back that is not significantly relieved by ibuprofen. Imaging shows a 3.5 cm lytic lesion with central mineralization in the posterior elements of T8. Histology demonstrates interlacing trabeculae of woven bone lined by prominent osteoblasts. What is the most likely diagnosis?

Explanation

The histology is identical to an osteoid osteoma, but the size (>2 cm), location in the spine's posterior elements, and lack of dramatic response to NSAIDs are classic for osteoblastoma. Unlike osteoid osteomas, osteoblastomas can be locally aggressive.

Question 57

A 12-year-old female with Neurofibromatosis Type 1 (NF1) presents with a rapidly progressive 45-degree thoracic scoliosis. Radiographs demonstrate "penciling" of three adjacent ribs, severe vertebral body scalloping, and pronounced rotation. What is the most appropriate surgical management?

Explanation

Dystrophic scoliosis in NF1 has a high risk of rapid curve progression and pseudarthrosis. An anterior and posterior spinal fusion is recommended to achieve solid arthrodesis and prevent progression.

Question 58

Which of the following accurately describes the genetic etiology of Neurofibromatosis Type 1 (NF1)?

Explanation

NF1 is an autosomal dominant disorder caused by a mutation in the NF1 gene on chromosome 17. This leads to a deficiency of the protein neurofibromin, resulting in overactivity of the Ras signaling pathway.

Question 59

A 4-year-old child presents with a congenital non-union of the tibia.

What is the typical direction of tibial bowing seen prior to fracture in patients with this condition?

Explanation

Congenital pseudarthrosis of the tibia in NF1 is characteristically preceded by anterolateral bowing. In contrast, posteromedial bowing is generally a benign physiological condition that typically resolves spontaneously.

Question 60

A 35-year-old male presents with a rapidly expanding, painful mass in his left thigh. He has a known history of NF1 and multiple plexiform neurofibromas. Biopsy reveals a high-grade spindle cell sarcoma. Which of the following is the most likely diagnosis?

Explanation

Patients with NF1 have up to a 10% lifetime risk of developing an MPNST. These highly aggressive tumors typically arise from the malignant transformation of a pre-existing plexiform neurofibroma.

Question 61

In the evaluation of primary pediatric bone tumors, which of the following cytogenetic abnormalities is considered pathognomonic for Ewing Sarcoma?

Explanation

Ewing sarcoma is characterized by the balanced translocation t(11;22)(q24;q12), which results in the EWS-FLI1 fusion protein. The t(X;18) translocation is characteristically seen in synovial sarcoma.

Question 62

A 28-year-old woman presents with a lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy shows mononuclear stromal cells and multinucleated giant cells. Which targeted therapy is most appropriate for extensive, unresectable disease?

Explanation

Giant Cell Tumor (GCT) of bone is characterized by RANKL expression by the neoplastic stromal cells. Denosumab, a monoclonal antibody against RANKL, is highly effective for unresectable or metastatic GCT.

Question 63

A 16-year-old boy presents with multiple non-ossifying fibromas (NOFs), café-au-lait macules, and intellectual disability. Genetic testing for NF1 is negative. What is the most likely diagnosis?

Explanation

Jaffe-Campanacci syndrome is characterized by multiple NOFs, café-au-lait spots, and intellectual disability. It mimics NF1 clinically but lacks the NF1 gene mutation.

Question 64

A 30-year-old female presents with a low-grade osteosarcoma located on the posterior surface of the distal femur. What gene amplification is characteristically identified in this tumor?

Explanation

Parosteal osteosarcoma typically occurs on the posterior aspect of the distal femur. It is molecularly characterized by the amplification of MDM2 and CDK4 on chromosome 12.

Question 65

A patient with NF1 undergoes an excisional biopsy of a symptomatic soft tissue mass. Histology reveals S-100 positive spindle cells with wavy nuclei in a myxoid stroma. What is the most appropriate classification of this tumor?

Explanation

The histology describes a benign neurofibroma, the hallmark lesion of NF1. It is composed of Schwann cells (S-100 positive), fibroblasts, and perineurial cells in a collagenous and myxoid matrix.

Question 66

Regarding Glucocorticoid-Induced Osteoporosis (GIO) in orthopedic patients undergoing chronic steroid therapy, which mechanism primarily drives the rapid early bone loss?

Explanation

GIO primarily results from direct inhibitory effects on bone formation. Glucocorticoids increase the apoptosis of mature osteoblasts and osteocytes while simultaneously decreasing new osteoblastogenesis.

Question 67

A 10-year-old boy is found to have a well-circumscribed, eccentrically located, lytic lesion with a sclerotic margin in the distal tibial metaphysis on an incidental radiograph. He is asymptomatic. What is the most appropriate management?

Explanation

The radiographic description is classic for a Non-Ossifying Fibroma (NOF). Asymptomatic NOFs are benign, self-limiting developmental defects that typically ossify over time and require only observation.

Question 68

A 5-year-old boy presents with six café-au-lait spots and axillary freckling. He is diagnosed with Neurofibromatosis Type 1 (NF1). The mutated gene in this condition normally functions to downregulate which of the following signaling pathways?

Explanation

NF1 is caused by an autosomal dominant mutation in the NF1 gene on chromosome 17. This gene encodes neurofibromin, a GTPase-activating protein that normally downregulates the Ras-MAPK pathway to prevent uncontrolled cellular proliferation.

Question 69

A 10-year-old female with Neurofibromatosis Type 1 presents with a rapidly progressive, short-segmented thoracic scoliosis. Radiographs reveal severe apical wedging, rib penciling, and evidence of dural ectasia. What is the most appropriate definitive management for this spinal deformity?

Explanation

Dystrophic scoliosis in NF1 has a very high rate of progression and pseudoarthrosis. Combined anterior and posterior spinal fusion is the standard of care to prevent pseudoarthrosis, hardware failure, and continued curve progression.

Question 70

A 32-year-old female presents with a large, lytic epiphyseal lesion of the distal femur. Biopsy confirms Giant Cell Tumor of bone (GCT). If medical therapy with denosumab is considered prior to surgery, which cellular interaction is primarily targeted?

Explanation

In GCT, the true neoplastic cells are the mononuclear stromal cells, which overexpress RANKL. Denosumab binds to this RANKL, preventing it from activating the RANK receptor on the reactive, bone-resorbing multinucleated giant cells.

Question 71

A 16-year-old male is undergoing treatment for localized high-grade conventional osteosarcoma of the proximal tibia. Following completion of neoadjuvant chemotherapy, surgical resection is performed. Which of the following findings is the most significant indicator of a poor overall prognosis?

Explanation

The degree of histologic tumor necrosis following neoadjuvant chemotherapy is the single most important prognostic factor in conventional osteosarcoma. Less than 90% necrosis indicates a poor chemotherapeutic response and correlates with lower overall survival.

Question 72

A 12-year-old boy presents with thigh pain, fever, and a large diaphyseal mass in the femur with an "onion skin" periosteal reaction. Core needle biopsy demonstrates sheets of small round blue cells. Which chromosomal translocation is most characteristic of this diagnosis?

Explanation

Ewing sarcoma classically presents with an "onion skin" periosteal reaction and small round blue cells on histology. It is definitively associated with the t(11;22)(q24;q12) translocation, resulting in the oncogenic EWSR1-FLI1 fusion protein.

Question 73

A 3-year-old girl with a known diagnosis of Neurofibromatosis Type 1 presents with the limb deformity shown in the provided clinical image.

This specific deformity most frequently progresses to which of the following complications if left untreated?

Explanation

Anterolateral bowing of the tibia is a hallmark of NF1 and frequently progresses to congenital pseudarthrosis of the tibia (CPT). This occurs due to a thickened, hamartomatous periosteum that strangles the blood supply to the underlying bone.

Question 74

A 28-year-old male with a history of Neurofibromatosis Type 1 presents with a rapidly enlarging and newly painful mass in his left thigh. He has had a long-standing, painless plexiform neurofibroma in this location since childhood. What is the estimated lifetime risk for patients with NF1 to develop a Malignant Peripheral Nerve Sheath Tumor (MPNST)?

Explanation

Patients with NF1 have an approximately 8-10% lifetime risk of developing an MPNST. A rapid increase in size or new onset of pain in a pre-existing plexiform neurofibroma is highly suspicious for malignant transformation.

Question 75

A 55-year-old male presents with deep pelvic pain. Imaging reveals a 12 cm destructive, permeative lytic lesion in the ilium with "popcorn" calcifications and cortical breakthrough. Biopsy confirms grade 3 conventional chondrosarcoma. What is the most appropriate primary treatment?

Explanation

Conventional chondrosarcoma is notably resistant to both traditional chemotherapy and radiotherapy. The standard of care for intermediate and high-grade conventional chondrosarcoma is wide surgical excision with negative margins.

Question 76

A 15-year-old male complains of right thigh pain that is significantly worse at night and dramatically improves 30 minutes after taking ibuprofen. Radiographs show a 1 cm radiolucent nidus surrounded by dense reactive sclerosis in the femoral diaphysis. What biochemical mediator is produced in excess by the central nidus, causing these classic symptoms?

Explanation

Osteoid osteomas classically present with night pain relieved by NSAIDs due to the exceptionally high levels of Prostaglandin E2 (PGE2) secreted by the central nidus. This robust PGE2 production drives both the intense pain and the surrounding reactive sclerosis.

Question 77

A 65-year-old man presents with back pain and hypercalcemia. Radiographs show multiple "punched-out" lytic lesions in the skull and vertebral bodies. A bone marrow aspirate reveals >10% clonal plasma cells. Which of the following tests is most appropriate to confirm the presence of the abnormal protein associated with this condition?

Explanation

Multiple myeloma is characterized by the neoplastic proliferation of plasma cells, leading to a monoclonal gammopathy. Serum and urine protein electrophoresis (SPEP/UPEP) detect the diagnostic M-spike and Bence-Jones proteins.

Question 78

A 14-year-old boy with a known history of Neurofibromatosis Type 1 undergoes a spine MRI due to worsening back pain and neurological symptoms, as shown in the provided clinical image.

Which of the following best describes the classical spinal manifestation demonstrated, which contributes to dystrophic spinal deformity in NF1?

Explanation

A classic dystrophic change seen in NF1 is posterior vertebral body scalloping, frequently caused by dural ectasia. This weakens the structural integrity of the spinal column and strongly predisposes the patient to severe, rapidly progressive scoliosis.

Question 79

A 9-year-old girl with Neurofibromatosis Type 1 presents with a rapidly progressive spinal curvature. Which of the following radiographic findings is most characteristic of dystrophic scoliosis in this patient?

Explanation

Dystrophic scoliosis in NF1 is characterized by short, sharp curves, severe apical vertebral wedging, rib penciling, enlarged foramina, and dural ectasia. These curves carry a high risk of rapid progression and often require early surgical spinal fusion.

Question 80

A 4-year-old boy presents with the clinical deformity shown in the radiograph.

What is the recommended management for a frank pseudoarthrosis of the tibia in a patient with Neurofibromatosis type 1?

Explanation

Congenital pseudoarthrosis of the tibia in NF1 is managed surgically once a frank pseudoarthrosis develops. Resection of the hamartomatous tissue, robust bone grafting, and rigid stabilization (often intramedullary) are required to achieve union.

Question 81

A 16-year-old male presents with severe knee pain. Radiographs reveal a destructive, lytic metaphyseal lesion in the distal femur. MRI shows multiple fluid-fluid levels. Biopsy reveals highly pleomorphic, atypical spindle cells producing delicate, lacelike osteoid. What is the most likely diagnosis?

Explanation

Telangiectatic osteosarcoma presents with large blood-filled spaces and fluid-fluid levels on MRI, mimicking an aneurysmal bone cyst (ABC). However, the presence of highly atypical, pleomorphic cells producing malignant osteoid confirms the diagnosis of osteosarcoma.

Question 82

A 12-year-old boy presents with an aggressive diaphyseal mass in his femur. Biopsy reveals small, round blue cells positive for CD99. Which of the following chromosomal translocations is most strongly associated with this tumor?

Explanation

Ewing sarcoma is classically associated with the t(11;22) translocation, resulting in the EWS-FLI1 fusion protein. CD99 (MIC2) is a sensitive but non-specific cell surface marker for this malignancy.

Question 83

A 45-year-old male with a history of multiple hereditary exostoses (EXT1 mutation) reports a newly growing mass on his proximal humerus. MRI shows a cartilage cap thickness of 3.5 cm. What is the most likely histological finding in this secondary malignancy?

Explanation

An enlarging osteochondroma with a cartilage cap greater than 2 cm in an adult is highly suspicious for secondary peripheral chondrosarcoma. Histology typically shows atypical chondrocytes, cellular pleomorphism, binucleation, and permeation into surrounding host bone.

Question 84

A 28-year-old woman with Neurofibromatosis Type 1 reports rapid enlargement and new onset severe resting pain in a long-standing thigh mass. MRI reveals a large, heterogeneous mass along the sciatic nerve with central necrosis. What is the most important prognostic factor for her likely diagnosis?

Explanation

Rapid growth and severe pain in a preexisting plexiform neurofibroma strongly suggest malignant peripheral nerve sheath tumor (MPNST). The most critical prognostic factor for survival is the ability to achieve a complete surgical resection with negative margins.

Question 85

A 32-year-old woman presents with a lytic epiphyseal lesion in her proximal tibia. Biopsy confirms Giant Cell Tumor (GCT) of bone. Because the tumor is deemed unresectable, medical therapy is initiated. The prescribed medication targets which of the following?

Explanation

Denosumab is a monoclonal antibody that targets RANKL, which is overexpressed by the neoplastic mononuclear stromal cells in GCT of bone. This prevents RANK activation on osteoclasts, dramatically reducing bone resorption and tumor progression.

Question 86

A 19-year-old male complains of severe, aching pain in his right shin that is worse at night and relieved by ibuprofen. Radiographs show a dense cortical thickening with a small 6 mm radiolucent nidus. What is the primary pain-generating mechanism in this lesion?

Explanation

Osteoid osteomas produce high levels of PGE2 via cyclooxygenase enzymes, causing intense local pain that is classically worse at night. This pathophysiology explains the rapid and dramatic pain relief provided by NSAIDs.

Question 87

A 65-year-old male presents with back pain and a pathological fracture of the L4 vertebral body. Laboratory studies reveal hypercalcemia and anemia. Which of the following is the most appropriate next step in confirming the suspected diagnosis?

Explanation

The clinical presentation of back pain, pathological fracture, hypercalcemia, and anemia in an older adult is highly suspicious for multiple myeloma. Serum and urine protein electrophoresis (SPEP/UPEP) with immunofixation are essential first steps to identify a monoclonal protein spike.

Question 88

A 55-year-old man presents with an impending fracture of his right femur secondary to a lytic metastatic lesion. His primary cancer is known to be renal cell carcinoma. Before performing prophylactic internal fixation, which of the following interventions is most strongly recommended?

Explanation

Metastatic renal cell carcinoma and thyroid carcinoma are notoriously hypervascular lesions. Preoperative embolization of the feeding vessels is strongly recommended to minimize catastrophic intraoperative blood loss during surgical stabilization.

Question 89

A pediatrician refers a 5-year-old child for an orthopedic evaluation of scoliosis. During the exam, you note multiple hyperpigmented macules on the child's trunk. According to the NIH diagnostic criteria for Neurofibromatosis Type 1, what is the minimum number and size of these macules required to meet this specific criterion in a prepubertal child?

Explanation

According to NIH criteria for NF1, a patient must have six or more cafe-au-lait macules. In prepubertal individuals, these macules must be greater than 5 mm in diameter; in postpubertal individuals, they must be greater than 15 mm.

Question 90

A 15-year-old male presents with knee pain. Radiographs reveal a 2 cm eccentric, well-circumscribed, lytic lesion with a thin sclerotic rim located entirely within the distal femoral epiphysis. Histology shows polygonal chondroblasts and "chicken-wire" calcifications. What is the most common complication following intralesional curettage of this tumor?

Explanation

Chondroblastomas are benign, epiphyseal cartilage tumors. While they can occasionally form secondary ABCs or rare "benign" pulmonary metastases, the most common complication following intralesional curettage is local recurrence (up to 15-20%).

Question 91

A 25-year-old male presents with a slowly growing, painful soft tissue mass near his knee joint. Radiographs show fine stippled calcifications within the soft tissue mass. Biopsy reveals a biphasic pattern of spindle cells and epithelial cells. What is the associated cytogenetic abnormality?

Explanation

Synovial sarcoma classically presents in young adults near large joints and often contains focal calcifications. It is defined by the t(X;18)(p11;q11) translocation, which fuses the SYT gene on chromosome 18 to one of the SSX genes on the X chromosome.

Question 92

A 9-year-old girl with Neurofibromatosis Type 1 (NF1) presents with a rapidly progressive thoracic scoliosis. Radiographs demonstrate a short-segmented, sharply angulated curve with vertebral body scalloping, severe apical wedging, and rib penciling.

What is the most appropriate surgical management for this patient's spinal deformity?

Explanation

Dystrophic scoliosis in NF1 is characterized by short, sharp curves, rib penciling, and vertebral scalloping. These curves have an extremely high risk of rapid progression and pseudoarthrosis, typically necessitating combined anterior and posterior spinal fusion for adequate, rigid stabilization.

Question 93

A 34-year-old male undergoes intralesional extended curettage and cementation for a Campanacci Grade III Giant Cell Tumor (GCT) of the distal femur. Because of extensive cortical thinning and soft tissue extension, he was treated with neoadjuvant denosumab for 6 months prior to surgery. Which of the following is the most clinically significant concern regarding the use of neoadjuvant denosumab followed by intralesional curettage in this setting?

Explanation

Denosumab effectively down-regulates osteoclast-like giant cells, leading to peripheral ossification and hardening of the tumor. However, this thickened neo-bone entraps microscopic neoplastic stromal cells, making adequate intralesional curettage difficult and paradoxically increasing local recurrence rates compared to curettage alone.

Orthopaedic Surgery Board Exam Review: ABOS Part I & AAOS OITE Prep Questions | Part 22210

Master ABOS Orthopedic Board Review: Bone Tumors, GIO & Neurofibromatosis | Part 1

Key Takeaway

Master ABOS Orthopedic Board Review: Bone Tumors, GIO & Neurofibromatosis | Part 1

Chapter Index