Among patients presenting with acanthosis nigricans, which is the most common anatomical site of involvement?

Explanation

## Distribution of Acanthosis Nigricans **Key Point:** The axillae (armpits) and inframammary folds are the most common sites of involvement in acanthosis nigricans, followed by the neck and groin. ## Characteristic Sites of Involvement | Site | Frequency | Clinical Features | |------|-----------|-------------------| | Axillae | Most common | Velvety, hyperpigmented plaques | | Inframammary folds | Most common | Symmetric involvement | | Neck (posterior/lateral) | Very common | "Dirty neck" appearance | | Groin/inguinal folds | Common | Often overlooked | | Palms/soles | Rare | Tripe palms (associated with malignancy) | | Dorsal hands/feet | Rare | Uncommon presentation | ## Pathophysiology of Site Predilection Accanthosis nigricans preferentially affects **intertriginous areas** (skin folds) because: 1. Increased friction and maceration in these regions 2. Higher concentration of insulin receptors on keratinocytes in flexural areas 3. Enhanced response to hyperinsulinemia in these zones 4. Moisture and heat promote epidermal hyperplasia **High-Yield:** The classic presentation is bilateral, symmetric involvement of the axillae and neck, giving a characteristic "dirty neck" or "velvety" appearance. ## Special Variants **Clinical Pearl:** **Tripe palms** (velvety thickening of the palms with exaggerated dermatoglyphics) is a rare variant strongly associated with malignancy, particularly gastric and lung cancers. This is distinctly different from typical acanthosis nigricans. **Warning:** Do not confuse tripe palms with typical acanthosis nigricans. Tripe palms on the palms and soles should raise suspicion for underlying malignancy, whereas typical acanthosis nigricans in intertriginous areas is usually metabolic. ## Clinical Examination Tips - Always examine the axillae, neck, groin, and inframammary folds - Look for symmetric involvement - Assess for associated features: hirsutism, obesity, acne (suggesting PCOS or metabolic syndrome) - Palpate for texture: velvety, thickened, hyperpigmented [cite:Fitzpatrick's Dermatology 9e Ch 73]