Paper I — 2024 December (Supplementary) (2019 Scheme) — Q3
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Paper I
2024 December (Supplementary) (2019 Scheme) · 100 marks · 180 min
Question
A 45 year old male presented in dermatology OPD with history of an itchy annular lesion over the scalp for
the past 20 days, which on examination looked like a fungal infection.
(a) What is the probable clinical diagnosis. 1 mark(s)
(b) What are the appropriate samples to be collected in the above case. 1 mark(s)
(c) Describe, in detail, the laboratory tests that must be done to reach a diagnosis. 5 mark(s)
(d) Enumerate three clinical types of superficial mycoses and their causative agent 3 mark(s)
Q310 marksEssays
Answer
a) Probable clinical diagnosis: Tinea capitis (a dermatophyte infection of the scalp) — an itchy, annular scalp lesion of 20 days’ duration with a fungal appearance is classical.
b) Appropriate samples: skin/scalp scrapings from the active, advancing edge of the lesion, and plucked hairs (not cut hairs, since the fungal invasion is within/around the hair shaft base) from the affected area.
c) Laboratory tests to reach a diagnosis
Wood’s lamp examination — some Microsporum species show characteristic green fluorescence of infected hair under UV light (an ectothrix infection pattern); most Trichophyton species (endothrix) do not fluoresce — a rapid bedside screening tool that also helps guide which hairs to sample.
Direct KOH (potassium hydroxide) microscopy — skin scrapings/plucked hairs are mounted in 10–20% KOH, which digests keratin, allowing visualization of septate, branching fungal hyphae and, on hair, arthroconidia arranged either around the hair shaft (ectothrix) or within it (endothrix) — the standard rapid first-line diagnostic test.
Culture — on Sabouraud dextrose agar (with cycloheximide/chloramphenicol to inhibit contaminating environmental fungi/bacteria), incubated at 25–30°C for up to 2–4 weeks; colony morphology, pigmentation (obverse and reverse), and growth rate provide presumptive genus identification.
Slide culture technique / microscopic examination of the culture (lactophenol cotton blue mount) — for definitive species identification based on the characteristic size/shape/arrangement of macroconidia and microconidia.
Molecular methods (PCR) — increasingly used in reference laboratories for rapid, precise species identification when needed.
d) Three clinical types of superficial mycoses and their causative agents
Tinea corporis (ringworm of the body) — caused by Trichophyton, Microsporum, or Epidermophyton species; annular lesion with central clearing and an active, scaly, advancing edge.
Tinea pedis (athlete’s foot) — most commonly caused by Trichophyton rubrum or T. mentagrophytes; interdigital scaling/maceration of the feet.
Pityriasis (tinea) versicolor — caused by Malassezia furfur; hypo/hyperpigmented scaly macules, particularly on the trunk, with characteristic “spaghetti and meatballs” appearance on KOH mount.
