From the name of this entity, you would sure think this is a fungal infection of the underarm hairs, but it isn't. It is a concretion of Corynebacterium bacteria which gives the unfortunate victim smelly underarms, stained tee shirts and a perplexing diagnostic dilemma. The picture shows normal hair shafts with interspersed hairs that are thicker, slightly yellowish, and distinctly different from their surrounding cousins.
This condition differentiates from white piedra, in that it is usually NOT nodular, which white piedra is. The treatment for this condition is to shave the underarms and use benzoyl peroxide.
BTW, saw a case today that you will see next week, after we get better pics. It will blow you away!!