OSCE — dystrophic toenails and a pigmented band: onychomycosis, psoriasis, and melanoma red flags

Brief (to candidate)

A 58-year-old man has yellow-brown crumbling great toenails for 2 years and asks for “the fungus tablet.” On exam you also notice a new single dark longitudinal band on the left thumbnail with pigment on the proximal nail fold. You have 8 minutes to triage nail dystrophies, confirm fungus before oral therapy, dose first-line treatment, and act on melanoma red flags.

Candidate instructions

1. Differentiate onychomycosis, nail psoriasis, and nail lichen planus clinically.
2. State why mycologic confirmation is required before oral antifungals.
3. Prescribe first-line oral terbinafine with duration and LFT monitoring.
4. Recognise single-band melanonychia ± Hutchinson's sign as possible subungual melanoma.
5. Name when nail matrix biopsy is mandatory.

Examiner checklist (mark each domain / 10)

Model key actions

• Confirm fungus before starting terbinafine 250 mg daily (12 weeks for toenails).^[2]
• Distinguish psoriasis/LP patterns so non-fungal dystrophy is not treated as onychomycosis.^[1][3]
• Escalate single-band melanonychia with Hutchinson's sign to matrix biopsy for melanoma exclusion.^[1]

Common errors

• Starting oral terbinafine on clinical “fungus nails” without KOH/culture/PCR.
• Missing nail LP pterygium and delaying steroids until permanent nail loss.
• Reassuring single-band melanonychia as racial/benign without red-flag assessment.
• Shave sampling pigment without matrix depth for Breslow-capable histology.
• Forgetting LFT counselling and concurrent tinea pedis treatment.