Getting to the bottom of it
This patient presented with dystrophic nails and the diagnosis was either psoriasis or a fungal infection. He was examined elsewhere for evidence of either of these conditions.
The only other sign of skin disease was this rash in the buttock cleft. This is typical of a flexural psoriasis with the rash confined to the occluded buttock surfaces. A fungal infection would extend out on to the buttock skin and have a scaly edge.
Flexural psoriasis like this is best treated with some Hydrozole cream, which is a mixture of 1% hydrocortisone and