Master Bone & Soft Tissue Pathology: CAT 2 OmpathStudy

PAPER INTRODUCTION Mount Kenya University — School of Medicine Programme: Bachelor of Medicine and Bachelor of Surgery, Year 3 Unit: Bone and Soft Tissue Pathology Assessment: Continuous Assessment Test (CAT 2), End of Semester 3 Two versions here: June 2025 CAT (30 MCQs — soft tissue/nerve/muscle) and June 2026 CAT (30 MCQs + 2 SAQs — bone/joint/histotech/oral pathology) Format: 30 MCQs, choose most correct answer, 22 minutes --- SET 1 — JUNE 2025 CAT Q1. A 45-year-old woman presents with a 5 cm, soft, mobile, painless subcutaneous mass on her thigh. Histology shows mature adipocytes with a thin fibrous capsule. What is the most likely diagnosis? A) Lipoma B) Hibernoma C) Conventional liposarcoma D) Epidermal inclusion cyst E) Dermatofibroma Answer: A — Lipoma Explanation: Classic benign fatty tumour — mature adipocytes, thin capsule, no atypia. Commonest soft tissue tumour overall. --- Q2. A 64-year-old man has a deep-seated thigh mass; biopsy reveals atypical adipocytes and amplification of the 12q13–q15 region including MDM2. Which subtype of liposarcoma is most likely? A) Myxoid liposarcoma B) Well-differentiated liposarcoma C) Pleomorphic liposarcoma D) Round-cell liposarcoma E) Dedifferentiated liposarcoma Answer: B — Well-differentiated liposarcoma Explanation: MDM2/CDK4 amplification at 12q13-15 is the molecular hallmark distinguishing this from benign lipoma — same-looking cells, but genetically malignant. --- Q3. A 25-year-old man develops a rapidly growing 3 cm forearm mass with a history of minor trauma. Histology shows plump fibroblasts, frequent mitoses, and a t(17;22) MYH9–USP6 fusion. What is the diagnosis? A) Nodular fasciitis B) Desmoid (deep fibromatosis) C) Fibrosarcoma D) Myositis ossificans E) Schwannoma Answer: A — Nodular fasciitis Explanation: Rapid growth + high mitotic activity mimics sarcoma, but it's a benign, self-limiting reactive proliferation. MYH9-USP6 fusion is diagnostic. --- Q4. A 30-year-old woman with familial adenomatous polyposis develops an infiltrative abdominal wall mass. Histology shows bland fibroblasts with nuclear β-catenin accumulation. What is the most likely lesion? A) Dermatofibroma B) Nodular fasciitis C) Desmoid tumour (deep fibromatosis) D) Solitary fibrous tumour E) Neurofibroma Answer: C — Desmoid tumour (deep fibromatosis) Explanation: APC mutation (FAP) → unopposed β-catenin accumulation in nucleus → fibroblast proliferation. Locally aggressive, doesn't metastasize. --- Q5. A 6-year-old child presents with a grape-like mass protruding from her vagina. Tumour cells are small, round, and show cross-striations on special stain. Which variant of rhabdomyosarcoma is this? A) Alveolar B) Embryonal (sarcoma botryoides) C) Pleomorphic D) Spindle cell/sclerosing E) Botryoid liposarcoma Answer: B — Embryonal (sarcoma botryoides) Explanation: Botryoid ("grape-like") variant of embryonal RMS occurs in vagina/bladder of young children. Cross-striations = skeletal muscle differentiation (desmin+, myogenin+). --- Q6. An adult presents with a deep retroperitoneal mass composed of interweaving fascicles of hyperchromatic spindle cells, positive for smooth muscle actin and desmin. What is the diagnosis? A) Rhabdomyosarcoma B) Leiomyoma C) Fibrosarcoma D) Leiomyosarcoma E) Angiosarcoma Answer: D — Leiomyosarcoma Explanation: SMA+/desmin+ spindle cells with atypia in a deep/retroperitoneal site = malignant smooth muscle tumour (leiomyomas are rarely retroperitoneal or this atypical). --- Q7. A 28-year-old man has a deep-seated knee mass present for several years. Histology shows uniform spindle cells in short fascicles and a t(X;18) SS18–SSX fusion. Which tumour is this? A) Synovial sarcoma B) Nodular fasciitis C) Liposarcoma D) Malignant peripheral nerve sheath tumour E) Desmoid tumour Answer: A — Synovial sarcoma Explanation: Despite the name, does NOT arise from synovium. t(X;18) SS18-SSX is pathognomonic. Occurs near joints in young adults. --- Q8. A 55-year-old patient has a large, high-grade pleomorphic spindle-cell sarcoma in the thigh with no identifiable line of differentiation. What is the most appropriate classification? A) Fibrosarcoma B) Undifferentiated pleomorphic sarcoma C) Leiomyosarcoma D) Liposarcoma E) Rhabdomyosarcoma Answer: B — Undifferentiated pleomorphic sarcoma Explanation: Diagnosis of exclusion — once IHC rules out muscle, fat, and other specific lineage markers, "no identifiable differentiation" = UPS (formerly called MFH). --- Q9. A patient with neurofibromatosis type 2 develops bilateral cerebellopontine-angle masses causing hearing loss. The lesions are encapsulated, S100-positive, and show Antoni A and B areas. What are these? A) Neurofibromas B) Schwannomas C) Meningiomas D) Ependymomas E) Metastatic carcinoma Answer: B — Schwannomas Explanation: Bilateral vestibular (CP A) schwannomas = hallmark of NF2 (distinct from NF1, which features neurofibromas). Antoni A (compact) and Antoni B (loose/myxoid) areas are classic. --- Q10. In pati