Hello, Welcome to HCM, I had gone through your history and understand your concern, Most common single little patch of skin in the folds on the back of the neck is may be acanthosis nigricans. It is most common in darker skinned individuals, and often asymptomatic and associated with medical conditions like diabetes and obesity. It can be a marker of a more serious condition, and should be evaluated by a doctor.Usually this skin change is gradual over months to years. Certain medications may also cause this thickening of the skin as a side effect. There is no specific testing that needs to be done on the skin changes other then a physical exam by your doctor, but you should be checked for diabetes and other metabolic disorders. I may suggest you to meet your Dermatologists he may offer procedures like dermabrasion and laser therapy to treat the skin changes. Thank you.