DPM Podiatric Pathology 3 — Questions and Answers
Question 1: Which dermatophyte species is most commonly responsible for tinea pedis (athlete's foot) in the United States?
- Trichophyton rubrum (Correct answer)
- Trichophyton mentagrophytes
- Epidermophyton floccosum
- Microsporum canis
Correct answer: Trichophyton rubrum
Trichophyton rubrum is the most common causative organism of tinea pedis worldwide, particularly the chronic moccasin-type presentation.
Question 2: A patient presents with a painful, pulsatile mass on the dorsum of the foot following a penetrating injury. Histology shows a false lumen lined by fibrous tissue without endothelium. What is this lesion?
- Pseudoaneurysm (Correct answer)
- True aneurysm
- Arteriovenous fistula
- Hemangioma
Correct answer: Pseudoaneurysm
A pseudoaneurysm (false aneurysm) forms when blood leaks from a vessel and is contained by surrounding fibrous tissue, lacking a true endothelial lining.
Question 3: Which pathological finding best distinguishes rheumatoid arthritis from osteoarthritis in a synovial biopsy of the ankle?
- Pannus formation with synovial hyperplasia and lymphocytic infiltrate (Correct answer)
- Cartilage fragments embedded in synovium
- Fibrocartilaginous metaplasia
- Calcium pyrophosphate crystal deposition
Correct answer: Pannus formation with synovial hyperplasia and lymphocytic infiltrate
Rheumatoid arthritis features pannus—a proliferative synovial tissue with lymphocyte and plasma cell infiltration that invades and destroys cartilage.
Question 4: A diabetic patient develops rapidly spreading necrosis of the foot with crepitus on palpation. Which pathological mechanism is primarily responsible?
- Gas production by anaerobic bacteria causing tissue emphysema (Correct answer)
- Arterial thrombosis with ischemic necrosis
- Venous gangrene from deep vein thrombosis
- Fungal hyphal invasion of dermal vessels
Correct answer: Gas production by anaerobic bacteria causing tissue emphysema
Necrotizing fasciitis and gas gangrene involve anaerobic bacterial species that produce gas as a metabolic byproduct, creating crepitus.
Question 5: Which histological pattern is most characteristic of a Morton's neuroma (interdigital neuroma)?
- Perineural fibrosis with epineurial thickening and demyelination (Correct answer)
- Schwann cell proliferation forming palisading nuclei
- Axonal regeneration with onion-bulb formation
- Necrosis of nerve fascicles with calcification
Correct answer: Perineural fibrosis with epineurial thickening and demyelination
Morton's neuroma is a degenerative condition showing perineural and endoneural fibrosis with demyelination, not a true neoplasm.
Question 6: A 65-year-old patient has a punched-out bone lesion of the calcaneus on X-ray. Biopsy shows sheets of plasma cells with clock-face chromatin. What is the diagnosis?
- Multiple myeloma (plasmacytoma) (Correct answer)
- Metastatic adenocarcinoma
- Ewing sarcoma
- Giant cell tumor
Correct answer: Multiple myeloma (plasmacytoma)
Plasma cell myeloma (plasmacytoma) presents as a lytic bone lesion composed of monoclonal plasma cells with characteristic clock-face nuclear chromatin.
Question 7: Which feature best differentiates a benign enchondroma from a low-grade chondrosarcoma in a bone biopsy of the foot?
- Permeation of cortical bone and entrapment of lamellar bone by tumor (Correct answer)
- Presence of hyaline cartilage lobules
- Endochondral ossification at the periphery
- Binucleated chondrocytes in lacunae
Correct answer: Permeation of cortical bone and entrapment of lamellar bone by tumor
Cortical bone permeation and entrapment of pre-existing lamellar bone by cartilaginous tumor indicate malignancy (chondrosarcoma) over enchondroma.
DPM Podiatric Pathology 3
Which dermatophyte species is most commonly responsible for tinea pedis (athlete's foot) in the United States?