Dermatology MCQ - Infiltrative and Neoplastic Disorders - Targetoid haemosiderotic naevus

A 25-year-old man presents with a solitary, 5-mm lesion on his thigh. He reports that the lesion appears as a flat, brown circle most of the time, but occasionally, after friction from clothing or trauma, it becomes raised, violaceous, and surrounded by a faint, ecchymotic halo, resembling a "target." This transient change resolves over several days. What is the most likely diagnosis and the key histopathological feature?

A. Cockade naevus, characterized by concentric rings of melanocytic nests.
B. Targetoid haemosiderotic haemangioma, characterized by dilated vascular spaces in the dermis with extravasated erythrocytes and haemosiderin deposition.
C. Melanoma, characterized by atypical melanocytes with pagetoid spread and dermal mitoses.
D. Dermatofibroma, characterized by a storiform pattern of spindle cells in the dermis.
E. Halo naevus, characterized by a dense lymphocytic infiltrate destroying nevus cells.

Correct Answer: B. Targetoid haemosiderotic haemangioma, characterized by dilated vascular spaces in the dermis with extravasated erythrocytes and haemosiderin deposition.

Answer and Explanation

The correct answer is B. This question describes the classic, fluctuating clinical presentation of a targetoid haemosiderotic haemangioma (also known as a Hobnail haemangioma). The key clue is the history of a stable, brownish lesion that intermittently becomes raised and violaceous with an ecchymotic ring following minor trauma. This is due to thrombosis and hemorrhage within the vascular spaces of the lesion. The histopathology confirms the vascular nature, showing dilated, thin-walled vessels with hobnail endothelial cells, extravasated red blood cells, and hemosiderin pigment from previous bleeding episodes.

Why the Other Options are Incorrect:

• A. Cockade naevus: While this lesion also has a "targetoid" appearance, its rings are composed of stable pigmentation due to melanocytes, not transient ecchymosis. Its appearance is fixed and does not fluctuate with trauma.

• C. Melanoma: This is the most critical entity to rule out. However, melanoma does not typically exhibit a transient, trauma-induced targetoid appearance that resolves spontaneously. A changing melanoma would not revert to a benign-looking state.

• D. Dermatofibroma: A dermatofibroma is a persistent, firm papule that may have a peripheral brown ring, but it does not undergo the dramatic, cyclical changes in color and elevation described. It dimples on lateral compression (Fitzpatrick's sign).

• E. Halo naevus: A halo naevus has a permanent, depigmented (white) halo, not a temporary, bruise-like (ecchymotic) one. The central nevus in a halo nevus typically regresses over time, which is not described here.

Additional High-Yield Information for Exams:

• Histopathology: The histology is diagnostic. Key features include:

  • Dilated, thin-walled vascular channels in the superficial and reticular dermis.

  • Hobnail ("matchstick") endothelial cells that project into the vascular lumina.

  • Extravasated erythrocytes and hemosiderin deposition within the dermis, evidence of previous hemorrhage.

  • A sparse lymphocytic infiltrate may be present.

• Differential Diagnosis: This is crucial due to the potential for clinical confusion with melanoma. The main differential includes:

  • Melanoma: The most important mimic. A history of cyclical change related to trauma is the key distinguishing clinical feature. Dermatoscopically, a targetoid haemosiderotic haemangioma may show a central reddish-brown to violaceous homogeneous area with a peripheral pale halo.

  • Angiokeratoma: A vascular lesion that is typically dark red to black and verrucous (warty) on the surface; it does not have a transient ecchymotic halo.

  • Kaposi's Sarcoma: Presents with multiple violaceous patches, plaques, or nodules, often on the lower extremities, but lacks the targetoid appearance and cyclical history.

• Prognosis: Excellent. This is a benign vascular tumor with no malignant potential.

• Management & Rationale:

  • Rationale: The primary goal is to establish the correct diagnosis and rule out melanoma, thereby avoiding an unnecessarily aggressive procedure.

  • First-line for a classic presentation: Clinical diagnosis with reassurance and monitoring can be considered. However, due to the significant clinical overlap with melanoma, a diagnostic biopsy (often a punch or excisional biopsy) is frequently performed and is the most prudent course of action.

  • Treatment: If the diagnosis is confirmed histologically, no further treatment is needed. If the lesion is bothersome to the patient, simple excision is curative.