Pitryiasis versicolor (aka Tinea versicolor aka Dermatomycosis furfuracea aka Tinea flaveaaka Liver spots aka Chromophytosis) points in favor - 1) it's versicolor i.e areas of hyper and hypopigmentation, let's call it dyspigmentation. Hypopigmentation has been attributed to azaleic acid produced by Malassezia species which causes competitive inhibition of tyrosinase and perhaps a direct cytotoxic effect on melanocytes. The cause for hyperpigmentation is not clear ( electron microscope - shows large melanosomes in hyperpigmented lesions and a thicker keratin layer ). 2) hardly any inflammation ( the other differentials i.e Seborrhoeic dermatitis, Pityriasis rosea, and Tinea corporis will have some visible inflammation esp. for such wide involvement ). additional evidence in the form of white scaling (under good light and magnification ) can be seen in P versicolor and it can be accentuated by scratching with a glass slide and helps to differentiate it from vitiligo and chloasma. point against: atypical location ( usually the lesions are found in the upper trunk and the arm flexures - areas rich in sebaceous glands ) note: erythrasma and P versicolor can coexist and may even mimic each other ( so do consider the possibility while treating ) Who is vulnerable ? The adolescent age group ( reasons same as acne ). Additionally in the tropics the farm laborers or others who work for long hours under the punishing tropical sun and wear tight undershirt / vest are particularly prone ( mechanism is easy to guess ) there is another population that is vulnerable - if somebody presents with asthenia and has multiple dermatoses +/- oral candidasis +/- lymphadenopathy , then think of HIV.
My differentials for this would be: Pityriasis versicolor Pityriasis alba Progressive macular hypomelanosis
