ABOS Board Review Part 16: Musculoskeletal Pathology

Question 1

A 45-year-old man presents with chronic knee pain and mechanical catching. Radiographs demonstrate multiple intra-articular calcifications of relatively uniform size.

Which of the following describes the underlying pathophysiology of this condition?

Explanation

Primary synovial chondromatosis is a benign condition characterized by synovial metaplasia leading to the formation of multiple cartilaginous nodules. These nodules often ossify and form uniform loose bodies within the joint space.

Question 2

A 24-year-old male presents with a painless, slow-growing nodule on the volar aspect of his distal forearm. Biopsy reveals a granuloma-like architecture with central necrosis and epithelioid cells. Immunohistochemistry shows a loss of INI-1 (SMARCB1) expression. Due to the high rate of a specific metastatic pathway, which of the following evaluations should be included in his staging?

Explanation

Epithelioid sarcoma classically presents as a superficial nodule in the distal extremities of young adults and shows loss of INI-1. It is notable among soft tissue sarcomas for having a high rate of regional lymph node metastasis, necessitating lymph node evaluation.

Question 3

A 22-year-old female undergoes marginal excision of an incidentally found thigh mass. Pathology demonstrates alveolar soft-part sarcoma (ASPS). In addition to a chest CT, which of the following imaging modalities is uniquely indicated for staging this specific sarcoma?

Explanation

Alveolar soft-part sarcoma (ASPS) has a unique propensity to metastasize to the brain, often even before pulmonary metastases are clinically apparent. Therefore, a brain MRI is a mandatory component of initial staging.

Question 4

A 35-year-old male presents with a slow-growing, painful mass near his knee joint. Biopsy reveals a biphasic tumor with both epithelial and spindle cell components. Cytogenetic analysis is most likely to show which of the following translocations?

Explanation

This clinical and histological presentation describes a synovial sarcoma. The pathognomonic cytogenetic abnormality for synovial sarcoma is the t(X;18) translocation, resulting in the SYT-SSX fusion gene. This drives the proliferation of both spindle and epithelial elements.

Question 5

A 42-year-old female presents with chronic knee pain, catching, and reduced range of motion. Radiographs demonstrate multiple stippled intra-articular loose bodies of relatively uniform size.

Which of the following describes the pathophysiology of this condition?

Explanation

The image and presentation are characteristic of primary synovial chondromatosis. This condition is caused by the benign metaplastic transformation of the synovial membrane into cartilage, which can ossify and become loose bodies. Treatment often involves synovectomy and loose body removal.

Question 6

A newborn presents with rhizomelic shortening of the limbs, frontal bossing, and midface hypoplasia. Radiographs show narrowing of the interpedicular distances in the lower lumbar spine. What is the genetic basis of this condition?

Explanation

This presentation is classic for achondroplasia, the most common form of short-limbed dwarfism. It is caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, which inappropriately inhibits chondrocyte proliferation in the growth plate.

Question 7

A 40-year-old male undergoes excision of a deep thigh mass. Pathology reveals a myxoid background, proliferating lipoblasts, and a distinct arborizing "chicken-wire" capillary network. Which of the following is true regarding this tumor's staging and behavior?

Explanation

The histology describes myxoid liposarcoma, characterized by the t(12;16) FUS-DDIT3 translocation. Uniquely among soft tissue sarcomas, it has a high propensity for extrapulmonary metastasis, particularly to the spine, warranting whole-spine MRI during staging.

Question 8

A 6-year-old boy with a history of multiple low-energy fractures, blue sclerae, and dentinogenesis imperfecta is started on intravenous pamidronate. What is the primary mechanism by which this medication increases bone mineral density in this patient?

Explanation

The patient has Osteogenesis Imperfecta (OI). Bisphosphonates like pamidronate work by inducing apoptosis in osteoclasts, thereby inhibiting bone resorption and allowing osteoblastic bone formation to increase overall bone mineral density.

Question 9

A 32-year-old male presents with a slowly enlarging, nodular skin lesion on his trunk that has been present for years. Biopsy reveals a storiform pattern of spindle cells that are CD34 positive. Which targeted therapy is FDA-approved for unresectable cases of this tumor?

Explanation

Dermatofibrosarcoma protuberans (DFSP) is characterized by a t(17;22) translocation leading to overexpression of Platelet-Derived Growth Factor B (PDGFB). Imatinib mesylate is a tyrosine kinase inhibitor that blocks the PDGF receptor and is highly effective for unresectable DFSP.

Question 10

A 9-year-old child presents with waddling gait, knee pain, and restricted joint motion. Radiographs reveal delayed, irregular ossification of the capital femoral epiphyses and a "double-layer" appearance of the patella on the lateral view. Mutations in which of the following genes are most commonly associated with this condition?

Explanation

Multiple Epiphyseal Dysplasia (MED) commonly presents with delayed, irregular ossification of epiphyses and a double-layer patella. The most common genetic mutation in MED involves the COMP gene, leading to abnormal chondrocyte extracellular matrix.

Question 11

A 15-year-old boy presents with a rapidly growing, deep soft tissue mass in his forearm. Histology shows aggregates of small round blue cells with central loss of cellular cohesion, resembling pulmonary alveoli. Which cytogenetic abnormality is most characteristic of this diagnosis?

Explanation

Alveolar rhabdomyosarcoma is an aggressive small round blue cell tumor common in adolescents. It is defined by the t(2;13) translocation resulting in the PAX3-FOXO1 fusion protein, which carries a worse prognosis compared to embryonal rhabdomyosarcoma.

Question 12

A 65-year-old female undergoes marginal excision of an unexpectedly malignant, deep 8 cm mass in her posterior thigh. Pathology demonstrates highly pleomorphic spindle cells with bizarre giant cells, frequent atypical mitoses, and no specific line of differentiation on immunohistochemistry. What is the most appropriate next step in management?

Explanation

The pathology describes Undifferentiated Pleomorphic Sarcoma (UPS), a high-grade soft tissue sarcoma. Because it was treated initially with a marginal excision (an "unplanned" excision), the standard of care is re-excision to obtain wide margins, typically combined with adjuvant radiation.

Question 13

An infant is born with severe micromelia, a "hitchhiker" thumb, clubfeet, and cystic swelling of the external ears that later calcify. Radiographs show a shortened first metacarpal. What is the primary biochemical defect in this disorder?

Explanation

Diastrophic dysplasia presents with a hitchhiker thumb, cauliflower ears, and severe clubfeet. It is caused by mutations in the SLC26A2 gene (DTDST), which codes for a sulfate transporter, leading to undersulfation of proteoglycans in the cartilage matrix.

Question 14

A 24-year-old male presents with a firm, painless subcutaneous nodule on his volar forearm that he noticed 6 months ago. It is slowly enlarging and now has secondary proximal nodular lesions along the arm. Biopsy shows nodular aggregates of epithelioid cells with central necrosis. Immunohistochemistry will most likely demonstrate a loss of which of the following?

Explanation

Epithelioid sarcoma typically affects young adults, presents as firm distal extremity nodules, and has a high propensity for proximal lymphatic spread. The hallmark immunohistochemical finding is the loss of INI1 (SMARCB1) expression.

Question 15

A 30-year-old female complains of recurrent knee effusions and pain. MRI shows a massive joint effusion with extensive synovial proliferation that exhibits low signal on both T1 and T2-weighted images due to hemosiderin. The pathogenesis of this condition is primarily driven by the overexpression of which of the following?

Explanation

Pigmented Villonodular Synovitis (PVNS), now called Tenosynovial Giant Cell Tumor (TGCT), is characterized by a t(1;2) translocation. This fusion causes an overexpression of CSF1, which recruits extensive numbers of non-neoplastic macrophages to form the tumor mass.

Question 16

A 10-year-old boy presents for evaluation of shoulder hypermobility. Examination reveals he can bring his shoulders together anteriorly in the midline. He also has a delayed eruption of permanent teeth. What transcription factor, essential for osteoblast differentiation, is defective in this condition?

Explanation

Cleidocranial dysplasia involves hypoplastic or absent clavicles, delayed closure of fontanelles, and supernumerary teeth. It is caused by a mutation in RUNX2 (formerly CBFA1), a master transcription factor required for osteoblast differentiation and intramembranous ossification.

Question 17

A 28-year-old female presents with a deep-seated, 4 cm mass bound to the Achilles tendon. Histology reveals nests of plump spindle cells with clear cytoplasm separated by fibrous septa. Immunohistochemistry is positive for HMB-45 and S-100. Which chromosomal translocation is pathognomonic for this tumor?

Explanation

Clear cell sarcoma, also known as melanoma of soft parts, typically involves tendons or aponeuroses of the foot and ankle in young adults. It is driven by the t(12;22) translocation resulting in the EWSR1-ATF1 fusion.

Question 18

A 4-year-old boy is brought to the clinic for short stature and a waddling gait. His parents note he appeared completely normal at birth and grew normally for the first two years. Examination reveals disproportionate short stature with relatively normal facial features and head circumference. Radiographs show small, irregular epiphyses and metaphyseal flaring. This condition is caused by a mutation in the gene encoding which protein?

Explanation

Pseudoachondroplasia presents with normal birth length and normal facial features, unlike true achondroplasia. It becomes apparent in early childhood and is caused by mutations in the COMP gene, similarly to Multiple Epiphyseal Dysplasia.

Question 19

A 50-year-old male is newly diagnosed with a high-grade pleomorphic liposarcoma of the posterior thigh measuring 12 cm. What is the most critical staging study required prior to definitive surgical intervention to assess for systemic disease?

Explanation

The most common site of metastasis for most high-grade soft tissue sarcomas of the extremity is the lungs. Therefore, a non-contrast CT scan of the chest is the standard and most critical imaging modality for systemic staging.

Question 20

A 25-year-old male presents with a rapidly growing, slightly tender, 2 cm mass on his volar forearm that appeared 3 weeks ago. He reports a minor contusion to the area 1 month ago. Excisional biopsy demonstrates a poorly circumscribed lesion with plump, rapidly proliferating fibroblasts in a myxoid stroma with extravasated red blood cells. There is brisk mitotic activity but no atypical mitoses. What is the most appropriate management?

Explanation

This presentation and histology describe nodular fasciitis, a benign, rapid proliferation of fibroblasts often mimicking sarcoma and linked to MYH9-USP6 fusions. It is often self-limiting and can spontaneously resolve; if completely excised via marginal biopsy, no further aggressive treatment is needed.

Question 21

A 45-year-old male presents with chronic knee pain and mechanical symptoms. Radiographs show multiple loose bodies of uniform size. An MRI shows a joint effusion with multiple cartilaginous nodules.

Which of the following is true regarding this condition?

Explanation

Primary synovial chondromatosis is a benign neoplastic process characterized by synovial metaplasia and clustered chondrocytes. Malignant transformation to chondrosarcoma is rare (less than 5%).

Question 22

A 55-year-old presents with a painless, rapidly enlarging thigh mass. An MRI demonstrates a deep 8 cm heterogeneous mass.

When planning a core needle biopsy, which principle must be strictly adhered to?

Explanation

The biopsy tract must be placed longitudinally and carefully planned so it can be excised en bloc with the tumor during definitive resection. Violating multiple compartments or using transverse incisions increases the risk of tumor seeding and complicates limb salvage.

Question 23

A 4-year-old child with frontal bossing, rhizomelic dwarfism, and trident hands is being evaluated. This condition is inherited in an autosomal dominant fashion, though most cases are sporadic. What is the underlying pathophysiology?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3, which leads to decreased chondrocyte proliferation in the proliferative zone of the physis. This results in the characteristic rhizomelic dwarfism.

Question 24

A 28-year-old male presents with a slowly growing, painful mass near his knee. Radiographs reveal a soft tissue mass with stippled calcifications. Biopsy demonstrates a biphasic pattern of spindle and epithelial cells. What is the characteristic cytogenetic abnormality associated with this tumor?

Explanation

Synovial sarcoma is characterized by the t(X;18) translocation involving the SYT and SSX genes. It commonly presents in young adults as a calcified periarticular mass and may exhibit a biphasic histologic pattern.

Question 25

A 6-year-old child with blue sclerae, dentinogenesis imperfecta, and multiple prior long bone fractures is scheduled for rodding of the femurs. Which of the following best describes the fundamental defect in this patient's condition?

Explanation

Osteogenesis imperfecta is caused by mutations in COL1A1 or COL1A2, leading to quantitative or qualitative defects in type I collagen. This results in extreme bone fragility, blue sclerae, and dentinogenesis imperfecta.

Question 26

An older adult presents with a massive, deep intramuscular mass in the posterior thigh. Biopsy shows mature adipocytes interspersed with atypical spindle cells and hyperchromatic stromal cells. Cytogenetic analysis reveals amplification of the MDM2 gene. Which of the following is the most likely diagnosis?

Explanation

Well-differentiated liposarcoma (atypical lipomatous tumor) is characterized by MDM2 and CDK4 gene amplifications on chromosome 12q. This distinguishes it from benign lipomas.

Question 27

A 40-year-old male is diagnosed with a large myxoid liposarcoma of the thigh.

During staging, which imaging study is specifically indicated for this histologic subtype, unlike most other soft tissue sarcomas?

Explanation

Myxoid liposarcoma has a unique propensity to metastasize to extrapulmonary soft tissues and bones, particularly the spine. Therefore, a whole spine MRI is recommended as part of the staging workup.

Question 28

A 7-year-old boy presents with short stature, waddling gait, and joint pain. Radiographs demonstrate delayed ossification of multiple epiphyses and a "double-layer" appearance of the patella. What gene is most commonly mutated in the autosomal dominant form of this condition?

Explanation

Multiple Epiphyseal Dysplasia (MED) is most commonly caused by a mutation in the COMP (Cartilage Oligomeric Matrix Protein) gene. The "double-layer" patella is a classic radiographic hallmark of this condition.

Question 29

A 65-year-old female undergoes marginal excision of a presumed benign subcutaneous cyst in her thigh at an outside facility. Final pathology reveals an undifferentiated pleomorphic sarcoma with positive margins. What is the most appropriate next step in management?

Explanation

Unplanned excisions ("whoops" procedures) of soft tissue sarcomas require subsequent wide local excision of the entire surgical bed to achieve negative oncologic margins. Radiotherapy is often utilized as an adjunct to improve local control.

Question 30

A newborn presents with open cranial sutures, absent clavicles, and delayed eruption of primary teeth. What is the fundamental mechanism of this patient's skeletal dysplasia?

Explanation

Cleidocranial dysplasia is caused by a mutation in the RUNX2 (CBFA1) gene, which is an essential transcription factor for osteoblast differentiation. It primarily affects intramembranous ossification, leading to hypoplastic clavicles and delayed cranial suture closure.

Question 31

A 45-year-old male presents with chronic knee pain, catching, and locking. Radiographs show multiple intra-articular calcified loose bodies of relatively uniform size.

What is the underlying pathophysiology of this condition?

Explanation

Synovial chondromatosis is a benign condition caused by metaplasia of the synovium into cartilage, which can later ossify. The uniform size of the loose bodies differentiates it from degenerative joint disease fragmentation.

Question 32

What is the most important prognostic factor for overall survival in a patient presenting with a localized high-grade soft tissue sarcoma of the extremity?

Explanation

Histologic grade is the most significant prognostic factor for overall survival and distant metastasis in soft tissue sarcomas. Size and depth are also important staging criteria but are secondary to grade in predicting survival.

Question 33

A 55-year-old female has an 8 cm deep, firm, painless mass in her anterior thigh.

A core needle biopsy is planned. Which of the following is an essential principle of the biopsy technique?

Explanation

The biopsy tract is considered contaminated with tumor cells and must be placed longitudinally so it can be excised entirely en bloc during definitive surgery. Transverse incisions complicate the final resection and may necessitate a larger soft tissue defect.

Question 34

A 4-year-old child with a known FGFR3 mutation presents for evaluation. Which of the following orthopedic complications is most associated with this condition?

Explanation

Achondroplasia (FGFR3 mutation) frequently presents with thoracolumbar kyphosis in infancy, which often resolves with ambulation but can persist. Foramen magnum stenosis is the key cervical issue, unlike the C1-C2 instability seen in Morquio syndrome or Down syndrome.

Question 35

A 30-year-old male presents with a slowly growing, painful mass near his knee. MRI shows a soft tissue mass with multi-locular fluid-fluid levels adjacent to the joint. Biopsy shows a biphasic pattern of epithelial and spindle cells. What is the characteristic chromosomal translocation?

Explanation

Synovial sarcoma is characterized by the t(X;18)(p11;q11) translocation, resulting in the SYT-SSX fusion gene. It often presents in young adults as a painful mass near (but rarely within) a joint.

Question 36

A 28-year-old female presents with a deep soft tissue mass in the foot attached to the plantar aponeurosis. Histology is positive for S-100 and HMB-45. This tumor shares its histological markers with which of the following?

Explanation

Clear cell sarcoma is also known as melanoma of soft parts and stains positive for melanin markers (S-100, HMB-45). It frequently involves the foot and ankle aponeuroses in young adults.

Question 37

A 7-year-old boy presents with waddling gait, knee pain, and short stature. Radiographs demonstrate delayed ossification of the capital femoral epiphyses bilaterally, mimicking Legg-Calve-Perthes disease, with a double-layered patella. What is the inheritance pattern and typical gene involved?

Explanation

Multiple epiphyseal dysplasia (MED) is typically autosomal dominant and frequently caused by a mutation in the COMP (Cartilage Oligomeric Matrix Protein) gene. A double-layered patella is a classic radiographic sign of this dysplasia.

Question 38

A 24-year-old female presents with a painless, slow-growing mass in her right thigh. Staging reveals multiple pulmonary metastases. Biopsy shows large cells with crystalline intracytoplasmic inclusions that are PAS-positive and diastase-resistant. What is the most likely diagnosis?

Explanation

Alveolar soft part sarcoma uniquely presents with PAS-positive, diastase-resistant intracellular crystals. It has a high rate of early metastasis, particularly to the lungs and brain, despite indolent growth.

Question 39

A 9-year-old girl presents with a proximal femur fracture. Radiographs show a "ground-glass" appearance in the proximal femur. She is noted to have precocious puberty and café-au-lait spots with irregular borders. Which mutation is responsible for this condition?

Explanation

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

Question 40

Which subtype of liposarcoma commonly occurs in the retroperitoneum or deep thigh, has a low propensity for distant metastasis, but frequently recurs locally if not completely excised?

Explanation

Well-differentiated liposarcoma (atypical lipomatous tumor) has minimal metastatic potential but tends to recur locally if incompletely excised. It characteristically features MDM2 and CDK4 amplification.

Question 41

A 25-year-old male presents with a non-healing, painless ulcer on the volar aspect of his forearm. The mass extends proximally along the lymphatic tracks. Which soft tissue sarcoma is most likely responsible for this presentation?

Explanation

Epithelioid sarcoma often presents as a superficial nodule or non-healing ulceration on the distal extremities of young adults. It is notable for its tendency to spread via regional lymphatics.

Question 42

A newborn presents with short-limb dwarfism, clubfeet, "hitchhiker" thumbs, and cystic swelling of the external ears. What is the underlying genetic defect?

Explanation

Diastrophic dysplasia is an autosomal recessive condition caused by a mutation in the DTDST (SLC26A2) gene, leading to defective sulfate transport. Classic signs include hitchhiker thumbs, cauliflower ears, and rigid clubfeet.

Question 43

A 60-year-old man underwent wide surgical excision of a 10 cm high-grade pleomorphic sarcoma of the deep posterior thigh. Final pathology margins were negative (R0). Which of the following adjuvant therapies is most indicated to maximize local control?

Explanation

For deep, high-grade, large (>5 cm) soft tissue sarcomas, adjuvant radiation therapy (pre- or postoperative) is the standard of care to maximize local control. It significantly decreases local recurrence rates compared to surgery alone.

Question 44

A 6-year-old child presents with frequent fractures, anemia, and cranial nerve palsies. Radiographs demonstrate uniformly dense bones with a "bone-within-bone" appearance. A defect in which cell type is primarily responsible for this disorder?

Explanation

Osteopetrosis is caused by defective osteoclast function, most commonly due to impaired carbonic anhydrase II or TCIRG1 mutations. This leads to failed bone resorption, resulting in dense but brittle bones and marrow obliteration.

Question 45

A 40-year-old male presents with a large soft tissue mass in his thigh. Biopsy shows a plexiform "chicken-wire" capillary network with lipoblasts. Which chromosomal translocation is pathognomonic for this tumor?

Explanation

Myxoid liposarcoma is characterized by the t(12;16) translocation resulting in the FUS-DDIT3 (TLS-CHOP) fusion gene. It is known to spread to extrapulmonary bony sites, such as the spine.

Question 46

A 35-year-old female presents with a nodular, plaque-like cutaneous mass on her trunk. Pathology reveals a "cartwheel" or "storiform" pattern of spindle cells staining positive for CD34. What is the surgical treatment of choice?

Explanation

Dermatofibrosarcoma protuberans (DFSP) is a locally aggressive tumor with high recurrence rates. It requires wide surgical margins (2-3 cm) or Mohs micrographic surgery to achieve negative margins and prevent recurrence.

Question 47

A 32-year-old woman presents with recurrent hemorrhagic effusions of the knee. MRI shows a villous synovial mass with "blooming artifact" on gradient-echo sequences due to hemosiderin. Which targeted therapy is FDA-approved for severe, refractory cases of this disease?

Explanation

Tenosynovial giant cell tumor (formerly PVNS) is driven by an over-expression of CSF1. Pexidartinib is a CSF1 receptor inhibitor FDA-approved for symptomatic, severe cases not amenable to surgical improvement.

Question 48

A 65-year-old male with a history of renal cell carcinoma presents with a lytic lesion in the proximal femur causing severe pain.

Prophylactic fixation is planned. What is a critical preoperative step specific to managing bony metastases from this primary tumor?

Explanation

Renal cell carcinoma (and thyroid carcinoma) bone metastases are highly vascular. Preoperative embolization is strongly recommended 24 to 48 hours prior to surgical stabilization to reduce massive intraoperative blood loss.

Question 49

A patient presents with absent clavicles, delayed closure of cranial sutures, and supernumerary teeth. What gene mutation is associated with this disorder?

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by mutations in the RUNX2 (formerly CBFA1) gene, which is a master transcription factor for osteoblast differentiation.

Question 50

A 45-year-old male presents with severe knee catching and locking. Radiographs and subsequent MRI reveal multiple calcific bodies within the joint space.

What radiographic feature best distinguishes primary synovial chondromatosis from secondary causes such as severe osteoarthritis?

Explanation

Primary synovial chondromatosis involves synchronous metaplasia of the synovium, resulting in loose bodies of uniform size. Secondary disease (often due to osteoarthritis) produces loose bodies of varying sizes due to sequential fragmentation of osteophytes.

Question 51

A 55-year-old male presents with a painless, deep, 8 cm mass in his posterior thigh. MRI shows a heterogeneous soft tissue lesion deep to the fascia.

Which of the following is the most appropriate next step in establishing a definitive diagnosis?

Explanation

Core needle biopsy is the standard of care for diagnosing soft tissue masses greater than 5 cm or deep to the fascia. It provides sufficient tissue for histologic architecture and molecular testing while minimizing the risk of track seeding.

Question 52

A 25-year-old male is evaluated for a slow-growing, painful mass adjacent to his knee joint. Histopathological analysis reveals a biphasic tumor consisting of both epithelial and spindle cells. What is the characteristic chromosomal translocation associated with this pathology?

Explanation

Synovial sarcoma is highly associated with the specific chromosomal translocation t(X;18), which fuses the SYT gene with an SSX gene. This molecular finding is diagnostic and drives tumor pathogenesis.

Question 53

A 10-year-old child presents with a waddling gait and knee pain. Radiographs demonstrate delayed, irregular epiphyseal ossification centers in the large joints, but the spine is entirely normal. Mutations in which of the following genes are most commonly responsible for the autosomal dominant form of this condition?

Explanation

Multiple Epiphyseal Dysplasia (MED) is characterized by irregular epiphyses with a normal spine. The most common cause of the autosomal dominant form is a mutation in the Cartilage Oligomeric Matrix Protein (COMP) gene.

Question 54

A 35-year-old female undergoes resection of a deep thigh mass. Histology demonstrates a prominent plexiform capillary network and lipoblasts in a myxoid stroma. In addition to the lungs, what is the most critical extrapulmonary site for metastasis screening in this specific tumor type?

Explanation

Myxoid liposarcoma has a unique and highly characteristic propensity to metastasize to the skeletal system, particularly the spine. Staging and follow-up must include whole-spine MRI or bone scans.

Question 55

A 4-year-old child presents with recurrent fractures following minimal trauma. Clinical examination reveals blue sclerae, normal intelligence, and mild bowing of the long bones. The underlying genetic defect primarily impairs which of the following?

Explanation

Osteogenesis Imperfecta (OI) is caused by mutations in the COL1A1 or COL1A2 genes. This leads to defective synthesis of type I collagen, resulting in brittle bones, blue sclerae, and dentinogenesis imperfecta.

Question 56

A 20-year-old female presents with a painless, slow-growing mass in her anterior thigh. Biopsy shows large cells with abundant eosinophilic cytoplasm and PAS-positive, diastase-resistant crystalline inclusions. Which of the following describes the typical clinical behavior of this tumor?

Explanation

Alveolar soft part sarcoma is notorious for its slow clinical growth but highly aggressive metastatic potential. It frequently metastasizes to the lungs and brain, requiring careful systemic staging.

Question 57

A neonate is diagnosed with Spondyloepiphyseal Dysplasia Congenita (SEDC). Clinical features include a short trunk, normal-length limbs, and a cleft palate. What is the most dangerous orthopedic complication associated with the cervical spine in this condition?

Explanation

SEDC is a type II collagenopathy associated with odontoid hypoplasia. This structural defect significantly increases the risk of severe atlantoaxial instability, potentially leading to catastrophic neurologic injury.

Question 58

A 32-year-old woman presents with a firm mass in the rectus abdominis muscle. Biopsy confirms aggressive fibromatosis (desmoid tumor) showing a clonal proliferation of uniform fibroblasts with abundant collagen. Intra-abdominal variants of this tumor are strongly associated with which of the following syndromes?

Explanation

Desmoid tumors (aggressive fibromatosis) arise sporadically but are strongly associated with Familial Adenomatous Polyposis (Gardner syndrome), particularly for intra-abdominal and mesenteric lesions.

Question 59

A 65-year-old male presents with a rapidly enlarging painless mass in the posterior thigh. Biopsy shows highly atypical, pleomorphic spindle cells with frequent atypical mitoses, but lacks specific differentiation.

Which of the following is true regarding Undifferentiated Pleomorphic Sarcoma (UPS)?

Explanation

Undifferentiated Pleomorphic Sarcoma (formerly malignant fibrous histiocytoma) represents a high-grade pleomorphic neoplasm. It is a diagnosis of exclusion made when a specific lineage of differentiation cannot be established.

Question 60

A newborn is diagnosed with short-limbed dwarfism, characteristic "hitchhiker" thumbs, clubfeet, and cauliflower ears. What is the underlying genetic mechanism of this condition?

Explanation

Diastrophic dysplasia is an autosomal recessive disorder caused by a mutation in the SLC26A2 gene. This impairs sulfate transport, leading to undersulfated proteoglycans in cartilage matrix.

Question 61

A 28-year-old male presents with a deep mass attached to the plantar fascia of the foot. Biopsy shows nests of polygonal cells with clear cytoplasm and melanin pigment. This tumor is a translocation-associated sarcoma. Which immunohistochemical marker is most likely positive?

Explanation

Clear cell sarcoma (melanoma of soft parts) is derived from neural crest cells and typically stains positive for melanocytic markers such as S-100, HMB-45, and Melan-A. It is driven by the t(12;22) translocation.

Question 62

A 22-year-old male presents with a painless, ulcerating nodule on his volar forearm, initially misdiagnosed as an infection. Biopsy confirms epithelioid sarcoma. What is a defining characteristic of the clinical behavior of this specific sarcoma?

Explanation

Epithelioid sarcoma is unique among soft tissue sarcomas because of its significant tendency to spread via regional lymph nodes. Clinically, it often presents in the distal extremities of young adults as a superficial, ulcerating mass.

Question 63

A 15-year-old male is referred for delayed tooth eruption and unusual shoulder mobility, allowing him to approximate his shoulders anteriorly at the midline. The primary genetic defect (CBFA1/RUNX2) in this condition impairs which of the following biological processes?

Explanation

Cleidocranial dysplasia involves a mutation in the CBFA1 (RUNX2) gene, an essential transcription factor for osteoblast differentiation. This leads to defective intramembranous ossification, affecting the clavicles and cranial vault.

Question 64

A patient with characteristic rhizomelic shortening and frontal bossing is diagnosed with achondroplasia. The underlying genetic mutation in FGFR3 exerts its effect on the growth plate by which of the following mechanisms?

Explanation

Achondroplasia is caused by an autosomal dominant gain-of-function mutation in FGFR3. This constitutive activation abnormally inhibits chondrocyte proliferation in the proliferative zone of the physis.

Question 65

A 6-year-old boy presents with a rapidly enlarging mass in his forearm. A biopsy confirms rhabdomyosarcoma. Which of the following features represents a favorable prognostic factor?

Explanation

In pediatric rhabdomyosarcoma, the embryonal subtype generally portends a better prognosis than the alveolar subtype. The alveolar subtype is often driven by the t(2;13) or t(1;13) translocations and is more aggressive.

Question 66

A 60-year-old female develops an aggressive soft tissue sarcoma in her axilla 10 years following lumpectomy and radiation for breast cancer.

According to Cahan's criteria for diagnosing radiation-induced sarcoma, which of the following must be met?

Explanation

Cahan's criteria state that a radiation-induced sarcoma must arise within the previously irradiated field, have a different histology from the primary tumor, and occur after a substantial latent period (typically >3-5 years).

Question 67

Radiographs of an infant reveal severe stippling of the epiphyses (chondrodysplasia punctata). A careful maternal history should be taken, as which teratogenic exposure produces a clinical phenocopy of this condition?

Explanation

Fetal warfarin syndrome mimics chondrodysplasia punctata (stippled epiphyses). Warfarin inhibits vitamin K-dependent proteins (like osteocalcin), severely disrupting normal bone mineralization during development.

Question 68

A 55-year-old male has a deep intramuscular fatty mass in the thigh. Biopsy shows mature adipocytes with occasional enlarged, hyperchromatic stromal cells. Amplification of which of the following genes is the molecular hallmark distinguishing this atypical lipomatous tumor from a benign lipoma?

Explanation

Atypical lipomatous tumors/well-differentiated liposarcomas are characterized by the amplification of the MDM2 and CDK4 genes on chromosome 12q. Benign lipomas typically lack this amplification.

Question 69

A 35-year-old female presents with a painless mass in her foot. Biopsy shows nests of pale-staining cells with prominent nucleoli. Immunohistochemistry is positive for S100 and HMB-45. What is the characteristic chromosomal translocation associated with this tumor?

Explanation

Clear cell sarcoma (melanoma of soft parts) characteristically presents in the foot or ankle of young adults. It is positive for S100 and HMB-45, and is driven by the t(12;22) EWS-ATF1 translocation.

Question 70

A 45-year-old male is diagnosed with myxoid liposarcoma of the proximal thigh. Which of the following imaging modalities is uniquely recommended for staging this patient prior to definitive management?

Explanation

Myxoid liposarcoma has a unique propensity to metastasize to other soft tissues and extrapulmonary bone sites, particularly the spine. A total spine MRI is routinely recommended for staging to rule out occult spinal metastases.

Question 71

A 6-year-old boy presents with short stature, severe clubfeet, and bilateral swelling of his external ears. Radiographs reveal progressive cervical kyphosis. Which of the following genes is mutated in this patient's condition?

Explanation

Diastrophic dysplasia is characterized by short-limb dwarfism, 'hitchhiker' thumbs, clubfeet, and cauliflower ears. It is caused by an autosomal recessive mutation in the SLC26A2 (DTDST) sulfate transporter gene.

Question 72

An 18-month-old child with a disproportionately large head, frontal bossing, and rhizomelic shortening of the limbs is diagnosed with a condition caused by a gain-of-function mutation in the FGFR3 gene. Which zone of the physis is directly inhibited by this mutation?

Explanation

Achondroplasia is caused by a gain-of-function mutation in FGFR3. This directly inhibits chondrocyte proliferation within the proliferative zone of the growth plate.

Question 73

A 30-year-old male presents with a deep thigh mass.

Biopsy confirms synovial sarcoma. Which of the following describes the most common chromosomal abnormality associated with this tumor?

Explanation

Synovial sarcoma is strongly associated with the t(X;18) translocation, resulting in the SYT-SSX fusion gene. It commonly presents as a deep, frequently calcified mass near a joint in young adults.

Question 74

A 12-year-old child presents with a mildly short stature and bilateral waddling gait. Radiographs demonstrate a double-layered patella and bilateral flattening of the femoral heads. What is the most likely genetic mutation?

Explanation

Multiple epiphyseal dysplasia (MED) most commonly results from a mutation in the COMP gene. A double-layered patella is a classic radiographic hallmark of this condition.

Question 75

Which of the following skeletal dysplasias is characterized by a defect in intramembranous ossification, resulting in delayed closure of cranial sutures, absent or hypoplastic clavicles, and coxa vara?

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (CBFA1) affecting intramembranous ossification. Hallmarks include open fontanelles, dental anomalies, and absent or hypoplastic clavicles.

Question 76

A 55-year-old male undergoes marginal excision of a massive, painless thigh mass. Histology demonstrates a pleomorphic spindle cell neoplasm arranged in a storiform pattern with giant cells. No specific translocations are found, and immunohistochemistry is non-contributory. What is the most likely diagnosis?

Explanation

Undifferentiated pleomorphic sarcoma (UPS), formerly known as malignant fibrous histiocytoma (MFH), is a diagnosis of exclusion. It classically presents with a storiform pattern and highly pleomorphic cells, lacking a specific fusion gene or distinct immunohistochemical marker.

Question 77

A 35-year-old male presents with a large, painless thigh mass. Biopsy confirms myxoid liposarcoma with the characteristic t(12;16) translocation. In addition to a chest CT, which of the following imaging modalities is most appropriate for staging this specific sarcoma?

Explanation

Myxoid liposarcoma has a unique propensity to metastasize to extrapulmonary sites, particularly the bone and spine. Therefore, whole-spine MRI or whole-body MRI is recommended for accurate staging to detect osseous metastases.

Question 78

An MRI of a 40-year-old female with chronic hip pain reveals multiple intra-articular loose bodies with a "ring and arc" pattern of calcification.

What is the most feared complication of long-standing primary disease of this type?

Explanation

Primary synovial chondromatosis involves benign cartilaginous metaplasia of the synovium. Rarely, long-standing cases can undergo malignant transformation into secondary chondrosarcoma, which should be suspected if there is rapid clinical deterioration or aggressive imaging features.

Question 79

A 30-year-old female presents with recurrent, bloody knee effusions. MRI shows a diffuse synovial process with blooming artifact on gradient-echo sequences. Biopsy confirms a condition characterized by a t(1;2) translocation. What is the mechanism of action of the targeted medical therapy recently approved for refractory cases of this disease?

Explanation

Tenosynovial giant cell tumor (formerly PVNS) is driven by a t(1;2) translocation causing Colony Stimulating Factor 1 (CSF1) overexpression. Pexidartinib, a CSF1R inhibitor, is approved for advanced or refractory cases.

Question 80

A 28-year-old female presents with a deep, slow-growing mass in the plantar aspect of her foot. Biopsy reveals a neoplasm with nested cells separated by fibrous septa. Immunohistochemistry is positive for S-100 and HMB-45. What is the characteristic chromosomal translocation associated with this tumor?

Explanation

Clear cell sarcoma (melanoma of soft parts) is characterized by the t(12;22) translocation resulting in the EWSR1-ATF1 fusion gene. It commonly presents in the distal extremities of young adults and stains positive for melanocytic markers.

Question 81

A 35-year-old male undergoes wide excision of a nodular skin plaque on his trunk. Pathology demonstrates a storiform pattern of spindle cells that strongly express CD34 but are negative for Factor XIIIa. Which targeted therapy is most appropriate for metastatic or unresectable forms of this tumor?

Explanation

Dermatofibrosarcoma protuberans (DFSP) is driven by a t(17;22) translocation resulting in COL1A1-PDGFB fusion. Unresectable or metastatic cases are treated with Imatinib, a tyrosine kinase inhibitor.

Question 82

Certain soft tissue sarcomas have a higher propensity for regional lymph node metastasis and require specialized staging. Which of the following soft tissue sarcomas most warrants evaluation of the regional lymph nodes?

Explanation

Sarcomas that frequently metastasize to lymph nodes can be remembered by the mnemonic SCARE: Synovial, Clear cell, Angiosarcoma, Rhabdomyosarcoma, and Epithelioid sarcoma. These tumors necessitate vigilant regional lymph node evaluation.

Question 83

A 26-year-old male presents with a deep mass near the knee joint. Biopsy reveals a biphasic tumor with both epithelial and spindle cell components. Cytogenetics confirm a t(X;18) translocation. Which of the following immunohistochemical markers is typically positive in the epithelial component of this tumor?

Explanation

Synovial sarcoma is characterized by the t(X;18) SYT-SSX translocation. Despite its name, it does not arise from synovium, and its epithelial component typically stains positive for Cytokeratin and Epithelial Membrane Antigen (EMA).

Question 84

A 50-year-old female presents with a slow-growing thigh mass. Biopsy reveals a "staghorn" vascular pattern and spindle cells. Immunohistochemistry is strongly positive for STAT6 and CD34. What is the specific molecular driver for this neoplasm?

Explanation

Solitary fibrous tumors (formerly hemangiopericytomas) are defined by an inversion on chromosome 12 leading to the NAB2-STAT6 fusion gene. This makes nuclear STAT6 expression a highly sensitive and specific immunohistochemical marker.

Question 85

A 4-year-old boy presents with progressive restricted neck movement and firm soft tissue swelling on his back. Physical exam reveals bilateral short great toes with hallux valgus. What is the most appropriate next step in management?

Explanation

Fibrodysplasia ossificans progressiva (FOP) is caused by an ACVR1 mutation and presents with congenital great toe malformations and progressive heterotopic ossification. Any surgical intervention or biopsy is strictly contraindicated as it will trigger explosive heterotopic bone formation.

Question 86

A 60-year-old male has an 8 cm high-grade undifferentiated pleomorphic sarcoma (UPS) resected from his anterior thigh. Pathology margins are reported as < 1 mm but negative. He receives postoperative adjuvant radiotherapy. What is the primary benefit of adding radiotherapy in this clinical scenario?

Explanation

Adjuvant radiotherapy in the treatment of soft tissue sarcomas significantly decreases the rate of local recurrence, particularly for large, high-grade, or deep tumors with close margins. However, it does not universally improve overall survival or prevent distant metastasis.

Question 87

A 35-year-old female with a history of numerous café-au-lait spots, axillary freckling, and Lisch nodules presents with rapid enlargement and new onset pain in a long-standing peripheral nerve mass. What is the most likely diagnosis of this newly changing lesion?

Explanation

Patients with Neurofibromatosis type 1 (NF1) have an increased risk of developing MPNSTs, which often arise from pre-existing plexiform neurofibromas. Rapid enlargement, new pain, or neurologic deficits in a stable mass strongly suggest malignant transformation.

Question 88

A 29-year-old postpartum female presents with a firm, painless mass in her anterior abdominal wall. Biopsy reveals a bland fibroblastic proliferation lacking cytologic atypia, with strong nuclear beta-catenin staining. According to current consensus guidelines, what is the most appropriate initial management?

Explanation

Desmoid tumors (aggressive fibromatoses) are locally aggressive but do not metastasize and are associated with beta-catenin mutations. Current consensus favors an initial "wait-and-see" approach (active surveillance), as many tumors remain stable or even spontaneously regress.

Question 89

A 35-year-old male presents with a slow-growing, painful mass on the plantar aspect of his foot. Radiographs demonstrate small stippled calcifications within the soft tissues. A core needle biopsy reveals a biphasic pattern of spindle cells and epithelial cells. Cytogenetic analysis of this tumor is most likely to demonstrate which of the following translocations?

Explanation

This is a synovial sarcoma, which frequently presents in the foot/ankle of young adults and may show calcifications on radiographs. It is characterized by the t(X;18) translocation resulting in the SYT-SSX fusion gene.

Question 90

A 45-year-old female presents with a deep intramuscular mass in her anterior thigh. Biopsy confirms the diagnosis of myxoid liposarcoma. Due to the unique metastatic behavior of this specific histologic subtype, which of the following additional imaging modalities is mandatory for accurate staging?

Explanation

Myxoid liposarcoma has a high propensity for extrapulmonary metastases, particularly to the bone and specifically the spine. Therefore, a whole spine MRI is uniquely recommended for staging this sarcoma.

Question 91

A 40-year-old male presents with chronic knee pain and locking. Imaging demonstrates multiple intra-articular radiopaque bodies. During arthroscopy, numerous cartilaginous nodules are found attached to the synovium. Which of the following best describes the underlying cellular process of this condition?

Explanation

Primary synovial chondromatosis is caused by the benign cartilaginous metaplasia of synovial intimal cells. These metaplastic nodules can detach and calcify or ossify within the joint space.

Question 92

A 30-year-old female presents with a deep 3 cm mass in her plantar aponeurosis. Biopsy reveals nests of pale cells with distinct nucleoli. Immunohistochemistry is strongly positive for S-100, HMB-45, and Melan-A. Given the diagnosis, staging should highly consider the risk of metastasis to which of the following sites?

Explanation

The diagnosis is clear cell sarcoma (melanoma of soft parts), characterized by the t(12;22) translocation and positive melanoma markers (S-100, HMB-45). Like epithelioid and synovial sarcomas, it has a high rate of regional lymph node metastasis.

Question 93

A 65-year-old male presents with a large, deep painless mass in his posterior thigh. Core needle biopsy confirms a well-differentiated liposarcoma. Molecular cytogenetic testing of the tumor cells is most likely to reveal amplification of which of the following?

Explanation

Well-differentiated and dedifferentiated liposarcomas are characterized by supernumerary ring chromosomes that lead to the amplification of MDM2 and CDK4 genes. This helps differentiate them from benign lipomas.

Question 94

A 28-year-old postpartum female complains of a firm, non-tender abdominal wall mass. Core biopsy reveals uniform, elongated spindle cells in a dense collagenous stroma without nuclear atypia. Immunohistochemistry shows nuclear staining for beta-catenin. What is the most widely accepted initial management for this lesion?

Explanation

The patient has a desmoid tumor (aggressive fibromatosis), characterized by beta-catenin (CTNNB1) mutations. Because of their unpredictable clinical course and tendency to stabilize or spontaneously regress, active observation is the preferred first-line management.

Question 95

A 6-year-old boy presents with a rapidly enlarging, painful mass over his scapula following a minor fall. Physical examination is notable for bilaterally shortened great toes with hallux valgus. Which of the following interventions is absolutely contraindicated in the initial management of this patient?

Explanation

The clinical presentation (scapular mass post-trauma and short great toes) is classic for Fibrodysplasia Ossificans Progressiva (FOP). Incisional biopsy or surgical trauma is strictly contraindicated as it will trigger explosive heterotopic ossification.

Question 96

A 35-year-old female presents with a recurrent bloody knee effusion. MRI demonstrates a nodular intra-articular mass with low signal intensity on both T1- and T2-weighted images, and a "blooming" artifact on gradient-echo sequences. Which targeted systemic therapy is FDA-approved for severe, unresectable cases of this disease?

Explanation

The diagnosis is Pigmented Villonodular Synovitis (PVNS), now termed Tenosynovial Giant Cell Tumor (TGCT), driven by a CSF1 translocation. Pexidartinib is a CSF1 receptor inhibitor approved for severe cases not amenable to surgical resection.

Question 97

A 40-year-old man presents with a slowly enlarging, nodular violaceous plaque on his chest wall. Pathology reveals a cellular spindle cell proliferation arranged in a dense "storiform" pattern, infiltrating deeply into the subcutaneous fat in a honeycomb pattern. What genetic aberration drives this neoplasm?

Explanation

The diagnosis is Dermatofibrosarcoma Protuberans (DFSP), characterized by its storiform histology and honeycomb infiltration of fat. It is driven by the t(17;22) translocation resulting in the COL1A1-PDGFB fusion protein.

Question 98

A 65-year-old male presents with a rapidly growing mass in his lateral thigh. He has a history of receiving external beam radiation therapy to the same extremity 15 years ago. Biopsy reveals highly pleomorphic spindle cells with bizarre multinucleated giant cells and atypical mitoses, but immunohistochemistry fails to show a specific line of differentiation. What is the most likely diagnosis?

Explanation

Undifferentiated pleomorphic sarcoma (UPS), formerly known as Malignant Fibrous Histiocytoma (MFH), is the most common soft tissue sarcoma in adults and the most frequent histologic subtype of radiation-induced soft tissue sarcoma.

Question 99

A 14-year-old boy presents with a rapidly expanding calf mass. Biopsy reveals a primitive small round blue cell tumor. Molecular testing confirms a t(2;13)(q35;q14) translocation producing a PAX3-FOXO1 fusion gene. Which of the following describes this condition?

Explanation

The t(2;13) and t(1;13) translocations are diagnostic of alveolar rhabdomyosarcoma. The alveolar subtype carries a significantly worse prognosis and higher rate of metastasis compared to the more common embryonal subtype.

Question 100

A 55-year-old male undergoes a limb-sparing marginal excision for a high-grade, 8 cm soft tissue sarcoma of the deep thigh. Final pathology confirms negative margins, with normal muscle measuring 1 mm at the closest point. Based on standard sarcoma treatment protocols, what is the most appropriate adjuvant therapy to minimize local recurrence?

Explanation

For large (>5 cm), deep, high-grade soft tissue sarcomas treated with limb-sparing surgery and close margins, perioperative (neoadjuvant or adjuvant) external beam radiation therapy is the standard of care to maximize local control.

Skeletal Dysplasias of the Spine - Arab Board MCQ Prep

Master ABOS Board Review: Musculoskeletal Pathology, Skeletal Dysplasias, Soft Tissue Tumors | Part 16

Key Takeaway

Master ABOS Board Review: Musculoskeletal Pathology, Skeletal Dysplasias, Soft Tissue Tumors | Part 16

Chapter Index