OSCE — rapidly growing painless nodule: Merkel cell carcinoma AEIOU and immunotherapy era

Brief (to candidate)

An 78-year-old fair-skinned man on long-term immunosuppression develops a rapidly expanding, painless red-violet nodule on the sun-exposed forehead over 6 weeks. Nodes are clinically negative. You have 8 minutes to apply AEIOU criteria, plan diagnosis/staging, and outline surgery, SLNB, radiotherapy, and immunotherapy principles.

Candidate instructions

1. Apply AEIOU criteria for Merkel cell carcinoma (MCC).
2. Plan biopsy + immunohistochemistry (CK20 perinuclear punctate; TTF-1 negative).
3. Outline wide local excision / Mohs and sentinel lymph node biopsy.
4. Mention adjuvant radiotherapy for high-risk disease.
5. State first-line anti-PD-1/PD-L1 for advanced/metastatic MCC.
6. Emphasise urgency (more aggressive than many melanomas clinically).

Examiner checklist (mark each domain / 10)

Model key actions

• Suspect MCC using AEIOU in an immunosuppressed elder with a rapidly growing painless nodule.^[2][6]
• Confirm with IHC (CK20 punctate / TTF-1−) then surgery + SLNB ± RT.^[5]
• Reserve systemic checkpoint immunotherapy as first-line for advanced disease.^[7][8]

Common errors

• Dismissing as “cyst/insect bite” because painless.
• Treating as ordinary SCC without neuroendocrine IHC panel.
• Omitting SLNB staging in clinically node-negative disease.
• Choosing multi-agent chemo first-line for metastatic MCC in the immunotherapy era.
• Missing immunosuppression as a major risk amplifier.