A 74-year-old smoker presented to his GP with cough and shortness of breath. Exam revealed pigmentation of the oral mucosa and also over the palms and soles. Tests show that he is diabetic and hypokalemic. What is the most probable dx?

The key is C. Ectopic ACTH. [The patient is smoker and probably developed small cell lung cancer which is working as a tumour producing ectopic ACTH. Hyperpigmentation is caused by high levels of circulating ACTH that bind to the melanocortin 1 receptor on the surface of dermal melanocytes. Ectopic ACTH induced raised cortisole is leading to diabetes and hypokalemia. A variant of Cushing's disease can be caused by ectopic ACTH production from a small-cell lung cancer].