A rash of conditions under the nappy
The rash corresponds to the area of contact from the wet nappy with sparing deep in the flexures. In this case the skin under the penis was not involved.
Other conditions to consider are seborrhoeic dermatitis and psoriasis, in which deep flexures are often involved. In all these nappy rashes there may be secondary candida, which generally presents as small pustules within the rash, or at the edge (satellite pustules). Another rare condition is acrodermatitis enteropathica or zinc deficiency. Typically the patient also has a perioral