Dermatology MCQ - Infiltrative and Neoplastic Disorders - Acquired melanocytic naevi

A 22-year-old woman presents with a long-standing, 4-mm, lightly pigmented, soft, papule on her forearm. It is symmetrical, has regular borders, and has not changed since she first noticed it in her teens. Which of the following best describes the expected histopathological correlate of this lesion?

A. Nests of melanocytes confined to the dermal-epidermal junction.
B. Nests of melanocytes located primarily within the papillary dermis.
C. Nests of melanocytes at the dermal-epidermal junction and within the underlying dermis.
D. A proliferation of single, atypical melanocytes with pagetoid spread throughout the epidermis.
E. Elongated rete ridges with an increased number of singly dispersed melanocytes.

Correct Answer: C. Nests of melanocytes at the dermal-epidermal junction and within the underlying dermis.

Answer and Explanation

The correct answer is C. This question describes a classic compound melanocytic naevus. The key clinical clues are the stable, symmetrical papule that appeared in childhood/adolescence. A papule (a raised lesion) indicates that there is a dermal component. Histologically, a compound naevus is defined by the presence of nests of melanocytes at the dermo-epidermal junction AND within the underlying dermis.

Why the Other Options are Incorrect:

• A. Nests of melanocytes confined to the dermal-epidermal junction: This describes a junctional naevus, which is typically a flat macule. Since this lesion is a raised papule, a purely junctional component is insufficient.

• B. Nests of melanocytes located primarily within the papillary dermis: This describes an intradermal naevus. These are very common, often more fleshy-colored and dome-shaped. While this is a possible diagnosis, the description of light pigmentation is more classically associated with a compound naevus, which retains a junctional (and thus pigmented) component. An intradermal naevus often loses its pigment over time.

• D. A proliferation of single, atypical melanocytes with pagetoid spread...: This describes melanoma in situ. The history of long-term stability and the classic benign features (symmetry, regular borders) rule this out.

• E. Elongated rete ridges with an increased number of singly dispersed melanocytes: This describes a lentigo. Lentigines are macules, not papules, and do not feature nests of melanocytes.

Additional High-Yield Information for Exams:

• Histopathological Evolution: This is a core concept:

  • Junctional Naevus: Nests of melanocytes at the dermo-epidermal junction. (Flat macule)

  • Compound Naevus: Nests at the junction AND in the dermis. (Raised papule)

  • Intradermal Naevus: Nests primarily or exclusively in the dermis. The melanocytes often become more neurotized (resemble nerve tissue) and less pigmented with time. (Dome-shaped, flesh-colored papule)

• Differential Diagnosis: The main differential for a small, pigmented papule includes:

  • Seborrhoeic Keratosis: "Stuck-on" appearance, waxy, with horn pseudocysts.

  • Dermatofibroma: Firm, dimples when pinched (Fitzpatrick's sign).

  • Basal Cell Carcinoma: Pearly, telangiectatic.

  • Blue Naevus: Dermal melanocytosis causing a blue-gray color.

• Prognosis: Acquired melanocytic naevi are benign. The vast majority have an extremely low malignant potential. The presence of a large number (>50-100) is a marker of an increased overall risk for melanoma, but the individual common naevi are not the precursors.

• Management & Rationale:

  • Rationale: For typical, stable lesions, the goal is reassurance and avoidance of unnecessary procedures.

  • First-line: Clinical diagnosis and reassurance. No further action is needed.

  • Excision/Biopsy: Indicated only if the lesion shows features of melanoma (the ABCDE criteria: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving or changing). Any changing or symptomatic lesion should be biopsied.