A 52-year-old man of Indian origin presents with dark, velvety thickening of the skin in the neck and axillae. On examination, he has skin tags and mild hirsutism. Fasting blood glucose is 180 mg/dL and HbA1c is 9.2%. What is the most common cause of acanthosis nigricans in this patient?

Explanation

## Most Common Cause of Acanthosis Nigricans **Key Point:** Type 2 diabetes mellitus is the most common cause of acanthosis nigricans, accounting for approximately 90% of cases in non-malignant acanthosis nigricans. ### Epidemiology and Association Acanthosis nigricans is strongly associated with insulin resistance. In this patient, the elevated fasting glucose (180 mg/dL) and HbA1c (9.2%) confirm poorly controlled type 2 diabetes. The presence of skin tags and hirsutism further support an insulin-resistant state. ### Pathophysiology Hyperinsulemia stimulates insulin-like growth factor (IGF) receptors on keratinocytes and fibroblasts, leading to: - Epidermal thickening (acanthosis) - Papillomatosis (velvety appearance) - Hyperpigmentation (melanin deposition) **High-Yield:** In Indian populations, type 2 diabetes is the predominant cause of acanthosis nigricans due to high prevalence of metabolic syndrome and insulin resistance. ### Clinical Presentation - **Sites:** Neck (most common), axillae, inframammary folds, groin, knuckles - **Appearance:** Dark brown to black, velvety, non-pruritic plaques - **Associated features:** Skin tags, hirsutism, obesity (as in this case) ### Malignancy-Associated vs. Metabolic | Feature | Metabolic (Diabetes) | Malignancy-Associated | |---------|---------------------|----------------------| | Onset | Gradual | Rapid (weeks to months) | | Age | Typically > 40 years | Variable, often > 50 years | | Associated signs | Skin tags, hirsutism | Paraneoplastic syndrome | | Prognosis | Chronic, stable | Poor (reflects cancer severity) | | Frequency | ~90% of cases | ~10% of cases | **Clinical Pearl:** When acanthosis nigricans develops rapidly or in an elderly patient without obvious metabolic disease, malignancy (especially gastric cancer in Asian populations) must be excluded. **Warning:** Do not assume acanthosis nigricans is always benign — a thorough metabolic and malignancy workup is essential, especially in high-risk populations. [cite:Fitzpatrick's Dermatology 9e Ch 95]