1. Acanthosis nigrican (AN) is usually found in body folds and is associated with diabetes, obesity (both are ruled out in your case), but also with hormonal disorder (hypothyroidism,problem with adrenal gland) or with use of drugs which have high high doses of niacin/steroids,. 2. were your hormonal profile,USg done? and whether there is any history of use of above said medication? 3. people with acanthosis nigricans are at risk of developing diabetes, so check your blood sugar level regularly, and also for any Cancer? 4. has anyone in your family suffered from this problem or from diabetes? 5. complete blood count, endoscopy,x-ray may be required to exclude cancer as a cause. 6. as told by the doctor to be a fungal INFECTION, it s totally wrong, neither it s an infection nor its contagious. 7. because AN is usually involved with change in skin color, no definite treatment is directed toward it, your health care provider has to look for any underlying disorder. 8. because it occurs on neck,elbow,hands,they can be a concern of embarrassment, self consciousness and probably low self esteem, thus medicated oils and lotions can be applied to lighten the marks. 9. treatment will depend upon the cause after further evaluation by your Physician, till then eat healthy,exercise regularly, and remain calm,cool and stress free.