A single, asymptomatic, non-palpable, round, 2 mm brown macule is present on the lower lip. What is the most likely diagnosis?

Correct answer: B. Focal melanosis (labial melanotic macule)

• A. Malignant melanoma would be expected to show at least some of the ABCDE features — asymmetry, irregular border, variegated colour, diameter over 6 mm, and evolution — and oral melanoma characteristically occurs on the HARD PALATE and MAXILLARY GINGIVA, not the lower lip. Any pigmented lesion that grows, ulcerates, or changes colour must be biopsied.
• B. CORRECT. Every descriptor points to a LABIAL MELANOTIC MACULE (focal melanosis): SINGLE, SMALL (under 1 cm), ROUND with regular borders, uniformly brown, completely FLAT (non-palpable) and asymptomatic. It results from increased melanin production by a normal number of basal melanocytes, is the most common pigmented lesion of the lip vermilion, and requires no treatment beyond documentation and periodic observation.
• C. A compound nevus is a nevus with both junctional and intradermal components, which makes it typically RAISED and PALPABLE. The stem explicitly states the lesion is non-palpable.
• D. Peutz-Jeghers syndrome produces MULTIPLE perioral and labial macules together with intestinal hamartomatous polyps and a raised risk of gastrointestinal malignancy. The stem describes a solitary lesion.

Note: Differential for oral pigmentation, sorted by number: SOLITARY — melanotic macule, nevus, amalgam tattoo, melanoma. MULTIPLE/DIFFUSE — physiologic racial pigmentation, smoker's melanosis, Addison disease, Peutz-Jeghers, drug-induced (minocycline, antimalarials), heavy metals.