Skin & Breast Pathology CAT: MKU Year 3 OmpathStudy

MOUNT KENYA UNIVERSITY SCHOOL OF MEDICINE BACHELOR OF MEDICINE AND BACHELOR OF SURGERY — YEAR 3 END OF SEMESTER 3 CONTINUOUS ASSESSMENT TEST (CAT) — JUNE 2026 MBPA 3536B SKIN AND BREAST PATHOLOGY Instructions: Attempt all questions. Write your registration number on every answer sheet. --- 1. A 30-year-old woman presents with a flat, non-palpable, hyperpigmented lesion on her cheek measuring 0.8 cm. No scale or induration. Which term best describes this lesion? a. Patch b. Macule c. Papule d. Plaque e. Nodule Answer: b Flat + non-palpable + <1 cm = macule (a patch is the same but 1 cm/larger flat lesion) 2. A 26-year-old man presents with a 1.5 cm raised, firm lesion with a flat top and well-defined borders on the forearm. No fluid component. Which morphological term is most appropriate? a. Papule b. Plaque c. Nodule d. Vesicle e. Wheal Answer: b Raised, flat-topped, well-demarcated, 1 cm , solid = plaque (a papule is the same morphology but <1 cm) 3. A 38-year-old woman presents with pruritic lesions following exposure to a new cosmetic product. Histology reveals epidermal spongiosis and vesicle formation. Most likely underlying mechanism? a. Type I hypersensitivity b. Type II hypersensitivity c. Type III hypersensitivity d. Type IV hypersensitivity e. Immune complex vasculitis Answer: d Allergic contact dermatitis = Type IV (delayed/cell-mediated) hypersensitivity — sensitized T cells react to hapten-modified proteins, causing spongiotic dermatitis 4. A 50-year-old man presents with erythematous scaly patches on sun-exposed areas. Histology shows atypical keratinocytes confined to the basal layer. Most likely diagnosis? a. Actinic keratosis b. Squamous cell carcinoma c. Basal cell carcinoma d. Psoriasis e. Seborrheic keratosis Answer: a Actinic keratosis = premalignant UV-induced lesion; atypia confined to the basal layer without full-thickness dysplasia (which would be SCC-in-situ/Bowen disease) 5. A 29-year-old woman presents with annular erythematous lesions with central clearing on the trunk. KOH prep reveals branching septate hyphae. Most likely diagnosis? a. Tinea corporis b. Pityriasis rosea c. Erythema multiforme d. Psoriasis e. Cutaneous candidiasis Answer: a Classic annular "ringworm" lesion + septate branching hyphae on KOH = dermatophyte infection = tinea corporis 6. A 61-year-old man presents with a pearly papule with telangiectasia on the nose. Histology shows nests of basaloid cells with peripheral palisading. Most likely diagnosis? a. Squamous cell carcinoma b. Basal cell carcinoma c. Melanoma d. Actinic keratosis e. Keratoacanthoma Answer: b Pearly, telangiectatic papule + basaloid cells with peripheral palisading is the classic description of basal cell carcinoma , the most common skin cancer 7. A 44-year-old woman presents with photosensitive rash over the cheeks sparing the nasolabial folds. Histology shows immune complex deposition at the dermoepidermal junction. Which lab test is most likely positive? a. Anti-desmoglein antibodies b. Anti-dsDNA antibodies c. Anti-BP180 antibodies d. Elevated IgE e. Rheumatoid factor Answer: b Malar rash sparing nasolabial folds + granular immunoglobulin/complement deposits at the dermoepidermal junction ( "lupus band" ) = SLE → anti-dsDNA is the characteristic serology (highly specific) 8. A 35-year-old man presents with grouped vesicles on the lips that recur periodically. Tzanck smear shows multinucleated giant cells. What is the causative agent? a. Varicella-zoster virus b. Herpes simplex virus type 1 c. Human papillomavirus d. Epstein-Barr virus e. Cytomegalovirus Answer: b Recurrent grouped vesicles on the lips ("cold sores") + Tzanck smear multinucleated giant cells = HSV-1 (VZV can give a similar Tzanck picture, but recurrent lip lesions specifically are classic HSV-1) 9. A 48-year-old diabetic patient presents with a rapidly progressive leg ulcer with black necrotic tissue and severe pain. Histology reveals extensive tissue necrosis and bacterial invasion along fascial planes. Most likely diagnosis? a. Cellulitis b. Erysipelas c. Necrotizing fasciitis d. Pyoderma gangrenosum e. Vasculitis Answer: c Rapid progression, severe/disproportionate pain, black necrotic tissue, spread along fascial planes in a diabetic = necrotizing fasciitis — a surgical emergency 10. A 27-year-old woman presents with target-like lesions on the palms and soles following a recent upper respiratory infection. Most likely diagnosis? a. Psoriasis b. Erythema multiforme c. Lichen planus d. Urticaria e. Dermatitis herpetiformis Answer: b Classic target/iris lesions on palms/soles, triggered by infection (commonly HSV or Mycoplasma) = erythema multiforme 11. A 53-year-old man presents with thickened, yellowish nails and scaling between the toes. KOH prep reveals fungal hyphae. Most likely diagnosis? a. Onychomycosis with tinea pedis b. Psoriasis c. Lichen planus d. Contact dermatitis e. Candidiasis Answer: a Yellow, thickened nails (onychomycosis) + interdigital scaling (ti