The image displays a characteristic dermatological finding in the axilla. Which of the following conditions is most likely represented?

Explanation

## Image Findings * Diffuse dark brown to black **hyperpigmentation** in the axillary region. * **Thickened skin** with a characteristic **velvety texture**. * **Accentuated skin folds** giving a somewhat **papillomatous** or verrucous appearance. * Presence of a small **skin tag** (acrochordon) on the lateral aspect. ## Diagnosis **Key Point:** **Acanthosis Nigricans** is characterized by symmetrical, velvety, hyperpigmented plaques, typically in intertriginous areas. The image clearly demonstrates the classic features of **Acanthosis Nigricans (AN)**. The **diffuse dark brown to black hyperpigmentation** combined with the **thickened, velvety texture** and **accentuated skin folds** in the axilla are pathognomonic. This appearance results from **epidermal hyperplasia** (acanthosis) and **hyperkeratosis**, leading to the thickened, velvety feel, along with increased melanin in the epidermis, causing the hyperpigmentation. AN is often a cutaneous marker for underlying systemic conditions. The presence of **skin tags** (acrochordons) is also a common associated finding, particularly in cases linked to insulin resistance. ## Differential Diagnosis | Feature | Acanthosis Nigricans | Tinea Corporis | Post-inflammatory Hyperpigmentation | Psoriasis Inversa | | :-------------------------- | :------------------------------------------------- | :------------------------------------------------- | :------------------------------------------------- | :------------------------------------------------- | | **Appearance/Texture** | Velvety, thickened, hyperpigmented (dark brown/black) | Erythematous, scaly, annular lesions, central clearing | Flat, macular, hyperpigmented patches, no thickening | Erythematous, shiny, macerated plaques, minimal scale | | **Location** | Intertriginous areas (axilla, neck, groin) | Anywhere on body, often trunk/limbs | Anywhere, follows inflammation/trauma | Intertriginous areas (axilla, groin, inframammary) | | **Symptoms** | Usually asymptomatic | Pruritic (itchy) | Asymptomatic | Pruritic, painful fissures | | **Associated Features** | Skin tags, obesity, insulin resistance, malignancy | Fungal infection, often with active border | History of rash, injury, or friction | Other psoriatic lesions, nail changes, arthritis | | **Key Differentiating Factor** | Velvety texture and diffuse hyperpigmentation | Annular, scaly, often itchy | Flat, macular, no thickening | Erythematous, shiny, non-scaly plaques | ## Clinical Relevance **Clinical Pearl:** Acanthosis Nigricans is a crucial cutaneous sign that prompts investigation for underlying metabolic disorders, most commonly **insulin resistance** (Type 2 Diabetes Mellitus, obesity, metabolic syndrome), and less commonly, internal **malignancy** (especially gastric adenocarcinoma). ## High-Yield for NEET PG **High-Yield:** The most common cause of Acanthosis Nigricans is **insulin resistance**, often associated with obesity. **Key Point:** Malignant Acanthosis Nigricans is typically more widespread, rapidly progressive, and often involves mucous membranes. It is most commonly associated with **gastric adenocarcinoma**. ## Common Traps **Warning:** Do not confuse the velvety hyperpigmentation of Acanthosis Nigricans with simple friction-induced hyperpigmentation or other inflammatory dermatoses. The characteristic **velvety texture** and **thickening** are key to diagnosis. ## Reference [cite:Rook's Textbook of Dermatology, 9th Edition, Chapter 49: Disorders of Pigmentation]