Acanthosis nigricans — SCE Dermatology MCQ

Correct answer: C — Insulin resistance

The correct answer is C, insulin resistance. Velvety hyperpigmented thickening of the neck folds and axillae with skin tags in a centrally obese man with impaired fasting glucose (8.1 mmol/L) is the classic phenotype of obesity related acanthosis nigricans, driven by hyperinsulinaemia that cross reacts with insulin like growth factor receptors on keratinocytes and fibroblasts, causing epidermal proliferation and pigmentation. This is the benign, common form seen in the context of the metabolic syndrome and pre diabetes/type 2 diabetes, distinct from the rare malignant form. The absence of red flag features (weight loss, dysphagia, abdominal pain) argues against an underlying gastrointestinal malignancy, and the gradual, symmetrical, flexural distribution with skin tags fits insulin resistance rather than a paraneoplastic process. Why the other options are wrong: E. Addison disease: causes diffuse hyperpigmentation of skin, mucosae and scars due to excess ACTH/MSH, not localised velvety flexural thickening, and is associated with weight loss and postural hypotension, not obesity and hyperglycaemia. B. Internal malignancy: malignant acanthosis nigricans is typically abrupt in onset, extensive, involves mucosae and palms (tripe palms), and presents with systemic symptoms such as weight loss, none of which are present here. A. Haemochromatosis: produces a bronze or slate grey generalised skin discolouration from iron deposition, alongside diabetes and arthropathy, not velvety flexural acanthosis. D. Chronic arsenic exposure: causes diffuse rain drop hyperpigmentation with palmoplantar hyperkeratosis and a history of occupational or environmental exposure, not the flexural pattern described. Key point: In an obese patient with hyperglycaemia, flexural acanthosis nigricans with skin tags points to insulin resistance, whereas abrupt, extensive or mucosal involvement with systemic symptoms should prompt investigation for underlying malignancy.

Reference: Primary Care Dermatology Society (PCDS), Acanthosis Nigricans, Clinical Guidance, https://www.pcds.org.uk/clinical-guidance/acanthosis-nigricans