Prompted by his new bride, who is concerned she might “catch something” from him, a 53-year-old man self-refers for evaluation of a slightly itchy intergluteal rash. He’s had it for years; it waxes and wanes but never fully resolves.
It has been previously diagnosed as a yeast infection, fungal infection, and even herpes. But none of the respective treatments have helped.
More history-taking reveals a family history of psoriasis (maternal grandmother), but the patient denies other areas of involvement or other skin changes. He also denies having arthritis.
A salmon-pink, 7-cm, roughly round dry patch covered by white tenacious scale is located in the upper intergluteal/sacral interface. There is no increased warmth or tenderness on palpation.
A similar process is noted in the periumbilical area (a difficult area for this patient to see, due to his weight). Inspection of his fingernails reveals 3/10 with definite tiny pits.
What is the diagnosis?