A 52-year-old man with type 2 diabetes mellitus presents with dark, velvety thickening of the skin over his neck, axillae, and groin. Histopathology shows acanthosis and papillomatosis. Regarding acanthosis nigricans, all of the following are true EXCEPT:

Explanation

## Acanthosis Nigricans: Pathology and Clinical Features ### Correct Answer Analysis **Option 1 (Histology)** is the FALSE statement. The key histological feature of acanthosis nigricans is **acanthosis (epidermal thickening) and papillomatosis**, NOT the absence of melanin increase. In fact, acanthosis nigricans typically shows: - Increased melanin in the basal layer - Hyperkeratosis (thickened stratum corneum) - Acanthosis (thickened stratum spinosum) - Papillomatosis (dermal papillae elongation) - Dermal fibrosis may be present, but melanin is usually increased, not absent ### Key Point: **The hallmark histology is acanthosis + papillomatosis with increased basal melanin, NOT melanin absence.** This distinguishes it from other hyperpigmented conditions. ### High-Yield Facts | Feature | Details | |---------|----------| | **Most common association** | Insulin resistance, obesity, type 2 DM (benign form) | | **Paraneoplastic form** | 5–10% of cases; gastric, lung, breast, ovarian cancers | | **Histology** | Acanthosis, papillomatosis, hyperkeratosis, **increased melanin** | | **Treatment** | Weight loss, metformin, topical retinoids, address underlying cause | ### Clinical Pearl: **Benign vs. Malignant Acanthosis Nigricans** - **Benign (90%)**: Associated with insulin resistance, responds to weight loss and glycemic control - **Malignant (5–10%)**: Rapid onset, widespread distribution, associated with internal malignancy; prognosis tied to cancer treatment ### Mnemonic: ACANTHOSIS NIGRICANS — INSULIN & CANCER - **A**canthosis (epidermal thickening) - **C**arcinoma-associated (paraneoplastic form exists) - **A**nd insulin resistance (most common benign cause) - **N**eck, axillae, groin (typical sites) - **T**hickened, velvety skin - **H**yperkeratosis on histology - **O**besity and metabolic syndrome - **S**kin darkening (hyperpigmentation) - **I**ncrease in melanin (not decrease) - **S**ymptomatic improvement with metformin [cite:Robbins 10e Ch 25]