Dermatology MCQ - Infiltrative and Neoplastic Disorders - Blue naevus

A 45-year-old woman presents with an asymptomatic, 4-mm, blue-black papule on the dorsal aspect of her wrist. The lesion has been present and unchanged for over a decade. It is firm on palpation and has a smooth, dome-shaped surface. Dermatoscopy reveals a homogeneous, structureless blue-black coloration. What is the fundamental histopathological characteristic that defines this lesion?

A. Nests of atypical melanocytes confined to the dermo-epidermal junction.
B. Heavily pigmented, dendritic melanocytes interspersed among the collagen bundles of the dermis.
C. A proliferation of thin-walled, dilated vascular channels in the mid-dermis.
D. A dense aggregate of lymphocytes and melanophages in the superficial dermis.
E. Cords and nests of epithelioid melanocytes with abundant pink cytoplasm.

Correct Answer: B. Heavily pigmented, dendritic melanocytes interspersed among the collagen bundles of the dermis.

Answer and Explanation

The correct answer is B. This question describes a classic blue naevus. The key clinical clues are the stable, blue-black color (due to the Tyndall effect), the location (dorsal hand/wrist is common), and the dome-shaped appearance. The fundamental pathological feature is the presence of dendritic (spindle-shaped), heavily pigmented melanocytes located deep in the reticular dermis, which are responsible for the blue color perceived through the overlying skin.

Why the Other Options are Incorrect:

• A. Nests of atypical melanocytes confined to the dermo-epidermal junction: This describes a junctional nevus or melanoma in situ. Blue nevi are dermal lesions and are not characterized by junctional nesting or significant atypia.

• C. A proliferation of thin-walled, dilated vascular channels...: This describes a vascular lesion such as a venous lake or hemangioma, not a melanocytic lesion.

• D. A dense aggregate of lymphocytes and melanophages...: While melanophages (macrophages that have ingested melanin) are common in regressing lesions or post-inflammatory hyperpigmentation, they are not the primary cell type in a blue naevus. The defining cell is the melanocyte itself.

• E. Cords and nests of epithelioid melanocytes with abundant pink cytoplasm: This describes a Spitz naevus. Spitz nevi are typically pink or tan, not blue-black, and are composed of large epithelioid cells, not deeply pigmented dendritic cells.

Additional High-Yield Information for Exams:

• Histopathology: There are variants, but the common blue nevus shows:

  • A well-circumscribed but unencapsulated focus in the mid to deep dermis.

  • Bipolar, dendritic melanocytes with long, branching processes, heavily laden with fine melanin granules.

  • The cells are interspersed between collagen bundles, often extending into the subcutaneous fat.

  • A variable number of melanophages are typically admixed.

  • The overlying epidermis is normal.

• Differential Diagnosis: The main differential for a blue-black papule/nodule includes:

  • Nodular Melanoma: The most critical mimic. Melanoma may be blue but often has other colors (red, white, pink), a history of change, and may ulcerate. Dermatoscopy of melanoma is typically more complex and heterogeneous.

  • Pigmented Basal Cell Carcinoma: Often has a rolled, pearly border and arborizing vessels on dermatoscopy.

  • Dermatofibroma: Firm, dimples on pinching, and often has a central white scar-like patch on dermatoscopy.

  • Cutaneous Metastasis: e.g., from melanoma. Usually has a history of a primary cancer and may be multiple.

• Prognosis: Common blue nevi are benign and have an extremely low malignant potential. The rare variant, cellular blue nevus, has a slightly higher (though still very low) potential for transformation into malignant blue nevus/melanoma.

• Management & Rationale:

  • Rationale: For a classic, stable lesion, the goal is to provide reassurance and avoid unnecessary procedures.

  • First-line for a classic lesion: Clinical diagnosis with reassurance. The combination of long-term stability and a classic dermatoscopic appearance (homogeneous blue pattern) is highly reliable for diagnosis.

  • Indication for Excision: Any atypical feature warrants excision for pathological diagnosis. This includes rapid change, increase in size, symptoms (pain, itching), or an atypical dermatoscopic pattern (e.g., heterogeneous blue color, white areas, ulceration).