Dermatology MCQ - Infiltrative and Neoplastic Disorders - Clear cell acanthoma

A 65-year-old man presents with a solitary, slow-growing, 1-cm plaque on his shin. The lesion is well-demarcated, moist-appearing, and has a delicate collarette of scale at its periphery. It has been present for several years without causing any symptoms. A dermatoscopic examination would most likely reveal which of the following patterns?

A. A central keratin plug with crown vessels
B. A homogeneous, blue-black structureless area
C. A "pearl necklace" or "string of pearls" pattern of vessels at the border
D. A red central lagoons with white lines
E. Arborizing telangiectasias

Correct Answer: C. A "pearl necklace" or "string of pearls" pattern of vessels at the border

Answer and Explanation

The correct answer is C. This question describes the classic clinical presentation of a clear cell acanthoma (CCA). The key clues are the location (lower leg), the slow-growing, solitary, well-demarcated plaque, and the characteristic "collarette" of scale. The pathognomonic dermatoscopic finding for CCA is the "string of pearls" pattern, which refers to regularly spaced, dotted or glomerular vessels arranged in a linear pattern along the edge of the lesion.

Why the Other Options are Incorrect:

• A. A central keratin plug with crown vessels: This describes a keratoacanthoma. The keratin plug is a central feature, and the "crown" or "crown of jewels" vessel pattern is characteristic.

• B. A homogeneous, blue-black structureless area: This is the classic dermatoscopic pattern of a blue nevus.

• D. Red central lagoons with white lines: This describes a vascular lesion, most classically a venous lake.

• E. Arborizing telangiectasias: This is the hallmark dermatoscopic feature of nodular basal cell carcinoma.

Additional High-Yield Information for Exams:

• Histopathology: The histology of a clear cell acanthoma is very distinctive:

  • A well-demarcated psoriasiform epidermal hyperplasia.

  • The key feature: the keratinocytes within the lesion have abundant, clear, glycogen-filled cytoplasm (PAS-positive, diastase-labile). These clear cells are sharply demarcated from the adjacent normal epidermis.

  • There is an absence of a granular layer within the lesion.

  • Neutrophils are often present within the epidermis.

• Differential Diagnosis: The clinical differential can include:

  • Psoriasis: But psoriasis is rarely a solitary, long-standing plaque.

  • Basal Cell Carcinoma: Especially the superficial type, but BCC lacks the "string of pearls" vessels and typically has more scale and erosion.

  • Dermatitis: Usually more eczematous and pruritic.

  • Pyogenic Granuloma: Tends to be more rapidly growing and friable.

• Associated Conditions & Prognosis:

  • Benign Nature: Clear cell acanthoma is a benign epidermal tumor.

  • Prognosis: Excellent. It is not known to have any malignant potential.

• Management & Rationale:

  • Rationale: The goal is to confirm the diagnosis, as its clinical appearance can be mistaken for other entities, including malignancy.

  • First-line/Definitive Management: A shave or punch biopsy is diagnostic. A complete excision is both diagnostic and curative.

  • Other Treatment Options: Given its benign nature, other superficial destructive modalities like cryotherapy, curettage, or laser ablation can be effective if the diagnosis is certain.