A 52-year-old Indian woman presents with progressive darkening and thickening of skin over her neck, axillae, and inframammary folds for the past 8 months. She reports associated weight gain of 12 kg over 2 years and persistent fatigue. On examination, the affected areas show velvety, hyperpigmented plaques with exaggerated skin markings. Fasting blood glucose is 186 mg/dL, and HbA1c is 9.2%. Abdominal ultrasound is normal. What is the most likely underlying pathophysiology of her skin findings?

Explanation

## Pathophysiology of Acanthosis Nigricans **Key Point:** Acanthosis nigricans is fundamentally a manifestation of **insulin resistance**, not a primary dermatological disease. The hyperinsulinemia drives paracrine signalling through IGF-1 receptors on skin cells. ### Mechanism 1. **Insulin resistance** → compensatory hyperinsulinemia 2. **Elevated insulin** binds to **IGF-1 receptors** (which have high homology to insulin receptors) on keratinocytes and dermal fibroblasts 3. This triggers: - Acanthosis (epidermal thickening) - Papillomatosis (velvety texture) - Hyperpigmentation (melanin deposition) ### Clinical Associations | Condition | Frequency | Notes | |-----------|-----------|-------| | Type 2 Diabetes Mellitus | 40–74% | Most common association | | Obesity | 5–10% of obese individuals | Often precedes diabetes diagnosis | | Polycystic Ovary Syndrome (PCOS) | 20–30% | Insulin resistance is central | | Acromegaly | Rare | IGF-1 elevation | | Malignancy (Paraneoplastic) | <5% | Abrupt onset, rapid progression | **High-Yield:** The presence of acanthosis nigricans should **always prompt screening for diabetes and metabolic syndrome**, even if fasting glucose is initially normal. It is a **clinical marker of insulin resistance**, not a consequence of hyperglycemia alone. **Clinical Pearl:** In this case, the patient's weight gain, fatigue, and elevated HbA1c confirm **Type 2 DM with insulin resistance** as the underlying cause. The normal abdominal ultrasound rules out malignancy-associated acanthosis nigricans (which would be paraneoplastic). ### Why Not the Other Options? - **Hyperglycemia toxicity:** Hyperglycemia causes glycation and oxidative stress, but does NOT directly activate IGF-1 signalling. Acanthosis nigricans can occur in insulin-resistant states with normal glucose levels. - **Autoimmune collagen destruction:** This describes scleroderma or systemic sclerosis, which present with fibrosis and induration, not the velvety papillomatosis of acanthosis nigricans. - **Fungal infection:** Would show scale, pruritus, and KOH-positive hyphae; lacks the characteristic hyperpigmentation and papillomatosis.