Dermatology MCQ - Infiltrative and Neoplastic Disorders - Benign melanocytic nevus of the nail matrix

A 25-year-old man presents with a 4-year history of a persistent, 4-mm-wide, brown longitudinal band on the thumbnail of his right hand. The band is uniform in color, and its pigmentation extends from the proximal nail fold to the free edge. There is no associated nail dystrophy, pain, or bleeding. Dermatoscopy reveals fine, parallel, brown lines of uniform color and spacing. There is no evidence of periungual pigmentation (Hutchinson's sign). What is the most appropriate initial management?

A. Perform an immediate punch biopsy of the nail bed to rule out acral lentiginous melanoma.
B. Obtain a radiograph of the digit to assess for a subungual glomus tumor.
C. Reassure the patient and schedule a follow-up examination in 6-12 months to monitor for stability.
D. Initiate treatment with a topical antifungal agent for a suspected fungal melanonychia.
E. Refer the patient for a full-thickness excision of the nail matrix.

Correct Answer: C. Reassure the patient and schedule a follow-up examination in 6-12 months to monitor for stability.

Answer and Explanation

The correct answer is C. This question describes a classic presentation of a benign melanocytic nevus of the nail matrix. The key reassuring features are the long history of stability, the young age of the patient, the uniform color of the band, and the dermatoscopic finding of regular, parallel lines. The absence of Hutchinson's sign (pigmentation in the proximal or lateral nail folds) is also a reassuring feature, though not absolute. In this clinical context, the most appropriate management is observation and periodic monitoring.

Why the Other Options are Incorrect:

• A. Perform an immediate punch biopsy...: This is incorrect and represents an overly aggressive approach for a lesion with multiple benign features. A biopsy is an invasive procedure that can permanently scar the nail matrix, leading to permanent nail dystrophy. It is reserved for lesions with suspicious features.

• B. Obtain a radiograph... for a glomus tumor: This is incorrect. A glomus tumor typically presents with a triad of severe, sharp pain, cold sensitivity, and point tenderness, not as an asymptomatic pigmented band.

• D. Initiate treatment with a topical antifungal...: This is incorrect. While fungal infection (onychomycosis nigricans) can cause melanonychia, it is uncommon and usually presents with other signs of nail dystrophy (e.g., thickening, crumbling). Topical antifungals have very poor penetration into the nail plate and are ineffective for this purpose. The dermatoscopic pattern described is not typical for fungus.

• E. Refer the patient for a full-thickness excision...: This is incorrect and overly aggressive. Excision is the treatment for a confirmed malignancy, not the first step for a clinically benign, stable lesion.

Additional High-Yield Information for Exams:

• Histopathology: A nevus of the nail matrix shows nests of benign melanocytes within the nail matrix epithelium. The melanocytes produce melanin, which is transferred to the growing nail plate, creating the longitudinal band.

• Differential Diagnosis (The "Mnemonic" for Melanonychia): The differential for a longitudinal pigmented band (melanonychia) is broad, but the critical distinction is from melanoma. Key entities include:

  • Benign Nail Matrix Nevus: Most common cause in children; stable, regular.

  • Melanocytic Activation (Physiological/Racial): Common in darker-skinned individuals; often multiple nails.

  • Drug-Induced Melanonychia: e.g., from chemotherapy agents.

  • Subungual Melanoma (Acral Lentiginous Melanoma): The diagnosis to rule out. Suspicious features include:

    • Hutchinson's sign: Pigmentation of the periungual skin.

    • Micro-Hutchinson's sign: Pigmentation of the cuticle visible only with dermatoscopy.

    • Triangular Band: Widening of the band as it grows out.

    • New onset in an adult (especially over 50).

    • Rapid change in color, width, or pattern.

    • Nail dystrophy (e.g., splitting, bleeding, ulceration).

    • Dermatoscopy: Irregular, disrupted lines with variable color (brown, black, gray).

• Prognosis: A benign nail matrix nevus has an excellent prognosis. The risk of transformation to melanoma is considered very low, though not zero.

• Management & Rationale:

  • Rationale: To avoid unnecessary, potentially scarring procedures on benign lesions while vigilantly screening for the rare but serious subungual melanoma.

  • First-line for a benign-appearing lesion: Clinical and dermatoscopic monitoring. Baseline photography and measurement of the band's width are essential for objective follow-up.

  • Indication for Biopsy: Any suspicious feature as listed above warrants a biopsy. The biopsy should be a longitudinal excision of the nail matrix to provide the best tissue for pathological diagnosis and to avoid sampling error.