## Malignancy Association in Acanthosis Nigricans **Key Point:** Acanthosis nigricans (AN) is a paraneoplastic syndrome in 90% of cases when it appears in adults. The underlying malignancy varies by geography and ethnicity. ### Geographic Variation | Region | Most Common Malignancy | Frequency | | --- | --- | --- | | **India & South Asia** | Gastric adenocarcinoma | 40–50% | | Western countries | Gastric & breast carcinoma | Variable | | Overall (global) | Gastric carcinoma | ~50% | **High-Yield:** In Indian populations, gastric cancer is the leading cause of paraneoplastic AN, followed by breast and lung malignancies. This reflects the higher incidence of gastric cancer in India due to H. pylori infection and dietary factors. ### Mechanism AN results from insulin-like growth factor (IGF) secretion by tumour cells, leading to: - Epidermal hyperplasia and papillomatosis - Increased melanin production - Velvety, hyperpigmented appearance of skin folds **Clinical Pearl:** When AN appears acutely in an adult without metabolic syndrome, malignancy screening (especially upper GI endoscopy in India) is mandatory. ### Other Associated Malignancies (in order of frequency) 1. Gastric adenocarcinoma (50%) 2. Breast carcinoma (10–15%) 3. Lung carcinoma (5–10%) 4. Ovarian, endometrial, pancreatic carcinomas (rare) **Warning:** Do not assume benign insulin resistance as the cause in an adult with acute-onset AN — malignancy must be excluded first. 
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