Dermatology MCQ - Infiltrative and Neoplastic Disorders - Cockade naevus

A 7-year-old girl is brought to the clinic for evaluation of a unusual "target-like" birthmark on her leg. On examination, there is a solitary, 1.5-cm lesion composed of a central, dark brown papule. This is surrounded by a concentric, flesh-colored zone, which is in turn bordered by a thin, complete, dark brown ring. The lesion is asymptomatic and has been present since infancy. What is the most accurate description of this entity?

A. A variant of a halo naevus in which the depigmented halo has begun to repigment in a ring-like pattern.
B. A combined nevus with a central blue nevus component and a peripheral compound nevus component.
C. A type of melanocytic nevus with a unique, concentric architectural pattern, often present in childhood.
D. An early stage of a congenital melanocytic nevus before it has fully pigmented.
E. A form of Meyerson's naevus with alternating rings of eczema and normal skin.

Correct Answer: C. A type of melanocytic nevus with a unique, concentric architectural pattern, often present in childhood.

Answer and Explanation

The correct answer is C. This question describes the pathognomonic appearance of a Cockade naevus (also known as a targetoid nevus). The key clinical clue is the "targetoid" or "rosette" appearance with concentric rings of pigmentation, typically a central pigmented papule, a hypopigmented or flesh-colored ring, and a peripheral pigmented ring. It is a rare variant of a melanocytic nevus that is often, but not always, congenital.

Why the Other Options are Incorrect:

• A. A variant of a halo naevus...: This is incorrect. A halo naevus features a zone of depigmentation (complete white loss of pigment), not a flesh-colored zone and a repigmented ring. The pathophysiology of halo nevi is immune-mediated destruction, which is different from the architectural patterning of a cockade naevus.

• B. A combined nevus with a central blue nevus...: This is an attractive but incorrect answer. While a combined nevus has two cell types, it does not typically arrange itself in such a perfect, concentric, targetoid pattern. The clinical description is far more specific for a cockade naevus.

• D. An early stage of a congenital melanocytic nevus...: This is incorrect. A small congenital melanocytic nevus is typically uniformly pigmented and does not develop a complex targetoid pattern as it matures.

• E. A form of Meyerson's naevus...: This is incorrect. Meyerson's naevus is characterized by an eczematous, erythematous, and scaly halo, not by concentric rings of pigmentation and normal skin.

Additional High-Yield Information for Exams:

• Histopathology: The histology correlates with the clinical appearance, showing a concentric architectural pattern. The central papule and the outer ring show nests of melanocytes at the dermo-epidermal junction and/or within the dermis (compound or intradermal nevus pattern). The intervening flesh-colored ring shows a marked decrease or absence of nevomelanocytes and pigmentation, corresponding to the dermal component "dropping off" or a area of normal skin.

• Differential Diagnosis: The appearance is so unique that the differential is narrow, but it could be mistaken for:

  • Atypical (Dysplastic) Nevus: These are often larger with irregular, "pebbled" pigmentation, but they lack the perfect concentric rings.

  • Melanoma: While melanoma can be multicolored, it rarely forms such a symmetrical, target-like pattern, especially in a child. Any sign of rapid change would be concerning.

  • Tinea Corporis: A fungal ringworm infection has a leading scaly edge and central clearing, but it lacks a central papule and the specific pigmented rings.

• Prognosis: Excellent. The Cockade naevus is a benign melanocytic lesion with no known increased malignant potential beyond that of a common acquired nevus.

• Management & Rationale:

  • Rationale: The goal is to correctly identify this benign entity to provide reassurance and avoid an unnecessary procedure.

  • First-line: Clinical diagnosis and reassurance. Due to its distinctive appearance, it can often be diagnosed confidently on sight.

  • Biopsy/Excision: Indications include diagnostic uncertainty, or if the lesion demonstrates atypical changes (e.g., rapid growth, irregularity within one of the rings, ulceration, or bleeding). For a classic, stable lesion in a child, excision is not required.