A 72-year-old fair-skinned woman presents with a slowly enlarging lesion on her lower leg that has been present for 2 years. On examination, the structure marked **A** — a well-demarcated erythematous plaque with adherent scale — is noted. Histopathology reveals full-thickness keratinocyte atypia with loss of normal maturation, abundant mitoses at all levels, and dyskeratotic cells, but the basement membrane remains intact. Which of the following best describes the pathological basis of this lesion?

Explanation

## Why option 1 is right The well-demarcated erythematous plaque marked **A**, combined with the histopathological findings of full-thickness keratinocyte atypia, abundant mitoses at all levels, dyskeratotic cells, and an INTACT BASEMENT MEMBRANE, is pathognomonic for Bowen disease (squamous cell carcinoma in situ). The defining feature is full-thickness epidermal dysplasia WITHOUT invasion of the dermo-epidermal junction. The clinical presentation on sun-exposed lower leg in an elderly fair-skinned woman and the 2-year indolent course are classic for Bowen disease (BAD Guidelines Cutaneous SCC In Situ 2014; Bolognia Dermatology 4th ed). ## Why each distractor is wrong - **Option 2**: Invasive SCC would show breached basement membrane and dermal invasion, which contradicts the intact basement membrane described in the histology. This represents progression beyond Bowen disease. - **Option 3**: Actinic keratosis shows only superficial (lower epidermis) atypia, not full-thickness dysplasia. The abundance of mitoses at all epidermal levels and dyskeratotic cells are not typical of AK. - **Option 4**: Superficial BCC presents with basaloid nests in the superficial dermis and typically lacks the full-thickness atypia and abundant mitoses seen here. The clinical presentation and histology do not fit BCC. **High-Yield:** Bowen disease = full-thickness epidermal atypia + intact basement membrane; the intact basement membrane is the key feature distinguishing it from invasive SCC. [cite: BAD Guidelines Cutaneous SCC In Situ 2014; Bolognia Dermatology 4th ed]