A 62-year-old woman presents with a solitary, slowly enlarging, erythematous papule with a moist, glistening surface on the lower leg. Shave biopsy is performed. Histologic sections reveal psoriasiform epidermal hyperplasia with pallor of the keratinocytes, parakeratosis, neutrophils in the stratum corneum, and numerous dilated blood vessels in the dermal papillae. The pale keratinocytes stain positively with PAS.

What is the most likely diagnosis?

A. Clear cell acanthoma

B. Bowen’s disease

C. Paget’s disease

D. Psoriasis

E. Tinea corporis

F. Basal cell carcinoma

Rationale:

The description of a solitary red plaque on the lower leg and the histologic findings of psoriasiform epidermal hyperplasia, keratinocyte pallor, neutrophils in the stratum corneum, and PAS-positive keratinocytes point toward a benign epithelial lesion with glycogen-rich clear cells. These features are classic for clear cell acanthoma.

Correct Answer: A. Clear cell acanthoma

Clear cell acanthoma (CCA) is a benign epidermal tumor that classically appears as a solitary, pink to red, glistening papule or plaque most commonly located on the lower extremities of older adults.

Histopathologic features include:

• Psoriasiform epidermal hyperplasia
• Pale (clear) keratinocytes due to glycogen accumulation
• Parakeratosis and neutrophils in the stratum corneum
• Dilated capillaries in the dermal papillae (explaining bleeding or "stuck-on" appearance)
• Positive Periodic acid-Schiff (PAS) staining due to intracellular glycogen
• No cytologic atypia or invasive growth

CCA is often confused clinically and histologically with psoriasis or superficial basal cell carcinoma, but the presence of glycogen-rich, PAS-positive clear keratinocytes is diagnostic.

Incorrect Answer Explanations:

B. Bowen’s disease (SCC in situ) typically shows atypical keratinocytes throughout the full thickness of the epidermis with disorganized architecture, mitotic figures, and no preservation of normal maturation. Clear cells are not a feature.

C. Paget’s disease shows large pale-staining cells (Paget cells) within the epidermis, often singly or in clusters, with mucin-positive cytoplasm. It most commonly occurs on the breast or genital area, not the leg.

D. Psoriasis shares several features (parakeratosis, neutrophils in the stratum corneum, psoriasiform hyperplasia), but does not show pale keratinocytes or PAS positivity for glycogen. It also typically presents with multiple symmetric plaques, not a solitary lesion.

E. Tinea corporis can show similar psoriasiform changes with neutrophils, but fungal elements would be present in the stratum corneum, highlighted by PAS stain. The keratinocytes themselves would not be PAS-positive due to glycogen, and fungal organisms would be identifiable.

 F. Although can appear clinically similar, histologically basal cell carcinoma BCC shows nests of basaloid cells, not clear keratinocytes, and is not typically PAS-positive for intracellular glycogen. It also does not present with a psoriasiform growth pattern or parakeratosis.

Additional reading at Barnhill's Dermatopathology Chapter 26: Tumors of the Epidermis