## Histopathology of Acanthosis Nigricans **Key Point:** The hallmark histopathological triad of acanthosis nigricans is: 1. **Acanthosis** — thickening of the epidermis 2. **Papillomatosis** — elongation and undulation of rete ridges 3. **Hyperkeratosis** — thickened stratum corneum ### Microscopic Features | Feature | Description | Significance | |---|---|---| | Acanthosis | Diffuse epidermal thickening | Proliferation of basal and suprabasal layers | | Papillomatosis | Exaggerated, finger-like projections of rete ridges | Creates the velvety, ridged clinical appearance | | Hyperkeratosis | Thickened stratum corneum | Contributes to hyperpigmentation and roughness | | Hyperpigmentation | Increased melanin in basal layer | Due to increased melanin transfer to keratinocytes | | Minimal inflammation | Usually absent or sparse | Distinguishes from other inflammatory dermatoses | **High-Yield:** The combination of acanthosis + papillomatosis + hyperkeratosis is pathognomonic for acanthosis nigricans and is a frequently tested histology question in NEET PG. **Mnemonic:** **APH** — **A**canthosis, **P**apillomatosis, **H**yperkeratosis. These three findings together define the histology of AN. ### Differential Histopathology - **Psoriasis:** Parakeratosis (nuclei in stratum corneum), munro microabscesses, dilated capillaries in dermal papillae - **Lichen planus:** Saw-tooth acanthosis, colloid bodies, band-like lymphocytic infiltrate - **Pemphigus vulgaris:** Suprabasal acantholysis with "tombstone" appearance - **Bullous pemphigoid:** Subepidermal bulla with eosinophil-rich infiltrate **Clinical Pearl:** The absence of significant inflammation on histology helps distinguish AN from inflammatory dermatoses like psoriasis or lichen planus, even when clinical appearance may overlap. 
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