Tan on his neck

A 48-year-old man visits his family physician, concerned about the tan on his neck. One month ago, he first spotted it, and since then, he has tried in vain to clean the area. Although he mentions that his garments are now looser than they were the previous year, he is generally healthy. Upon examination, the neck and axillae are covered in hyperpigmented, velvety plaques. His glycated haemoglobin (HbA1c) is 5.1%, and he has a body mass index of 22. An undetected cancer is considered, given the sudden onset of this hyperpigmentation and the absence of insulin resistance.