Deficiency of which one of the following dietary components is most likely to have caused this rash?

Answer C. NiacinThe symmetric, scaly, sunburn-like, hyperpigmented rash extending from the hands to a clearly demarcated border midway up the arm suggests a photosensitive distribution, most consistent with pellagra (niacin deficiency). Deficiency of the other listed dietary components is not typically associated with a photosensitive dermatitis in this area.