Dermatology MCQ - Infiltrative and Neoplastic Disorders - Pigmented melanotic macules

A 32-year-old woman presents with a new, asymptomatic, 4-mm, uniformly dark brown macule on her lower vermilion lip border. It appeared six months ago and has remained stable. There is no history of trauma or bleeding. The most likely diagnosis and corresponding histopathological finding is which of the following?

A. Labial melanotic macule, characterized by melanin deposition in the basal layer with a normal or slightly increased number of melanocytes.
B. Oral melanocytic nevus, characterized by nests of benign melanocytes at the junction and/or within the lamina propria.
C. Peutz-Jeghers type lentigo, characterized by elongated rete ridges and melanocytic hyperplasia in the context of gastrointestinal hamartomas.
D. Amalgam tattoo, characterized by deposition of fine, dark granules within the connective tissue, visible with polarization.
E. Oral melanoma in situ, characterized by confluent pagetoid spread of severely atypical melanocytes throughout the epithelium.

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Correct Answer: A. Labial melanotic macule, characterized by melanin deposition in the basal layer with a normal or slightly increased number of melanocytes.

Answer and Explanation

The correct answer is A. This question describes a classic labial melanotic macule. The key clinical clues are the location (vermilion border), the small size, uniform color, and stability. These are the most common pigmented lesions of the lip.

Histopathologically, a labial (or oral) melanotic macule is defined by:

• Prominent basal layer hyperpigmentation due to increased melanin in keratinocytes.

• A normal or only slightly increased number of melanocytes. This is the critical feature that distinguishes it from a melanocytic nevus or melanoma.

• The rete ridges are typically not elongated, unlike in a cutaneous lentigo.

• There is no nesting of melanocytes or significant cytologic atypia.

Why the Other Options are Incorrect:

• B. Oral melanocytic nevus...: This is incorrect. While intraoral melanocytic nevi occur, they are much less common on the vermilion border than melanotic macules. The histology would show nests of melanocytes, which are not a feature of a melanotic macule.

• C. Peutz-Jeghers type lentigo...: This is incorrect. While Peutz-Jeghers syndrome presents with labial macules, the diagnosis requires the presence of associated gastrointestinal polyps. A solitary, new macule in an adult without a relevant systemic history is most likely an isolated labial melanotic macule. Histologically, Peutz-Jeghers macules can be identical to solitary macules.

• D. Amalgam tattoo...: This is incorrect. An amalgam tattoo is caused by traumatic implantation of dental filling material and appears as a blue-gray or black, flat macule within the oral mucosa, not typically on the vermilion border. Histology shows foreign material, not melanin.

• E. Oral melanoma in situ...: This is incorrect and represents a worst-case scenario. Oral melanomas are rare and typically present as larger, irregularly pigmented, and enlarging plaques or nodules, often with a history of change. The histology shows severe melanocytic atypia and pagetoid spread, which is not present in a benign macule.

Additional High-Yield Information for Exams:

• Histopathology: The core finding is hyperpigmentation of the basal keratinocytes without melanocytic proliferation (no nests, no significant increase in melanocyte number). There may be melanin in the upper dermis (pigmentary incontinence), especially in older lesions.

• Differential Diagnosis: This is crucial. The main differential for a pigmented oral/labial macule includes:

  • Oral Melanotic Macule (as above): The most common diagnosis.

  • Amalgam Tattoo: Blue-gray color, often adjacent to a filled tooth.

  • Physiological Pigmentation: Diffuse, symmetric pigmentation, common in darker skin types.

  • Drug-Induced Pigmentation (e.g., from antimalarials, minocycline).

  • Melanocytic Nevus: Less common; requires histology to confirm nests.

  • Oral Melanoma: A sinister diagnosis; lesions are typically >6mm, asymmetric, with color variegation and a history of change.

• Prognosis: Excellent. Labial and oral melanotic macules are benign with no significant malignant potential. However, any changing or atypical oral pigmented lesion warrants biopsy to rule out melanoma.

• Management & Rationale:

  • Rationale: To confirm the benign diagnosis and rule out malignancy, especially if the lesion is new, changing, or has any atypical features.

  • First-line for a classic, stable lesion: Clinical diagnosis with observation and reassurance is appropriate.

  • Indication for Biopsy: Any doubt in the clinical diagnosis, or if the lesion has features of melanoma (e.g., rapid growth, ulceration, color variegation, diameter >6mm). A shave or punch biopsy is sufficient for diagnosis.

  • Treatment: No treatment is medically necessary. If desired for cosmetic reasons, lasers (e.g., Q-switched) can be effective.