Tough to say without seeing it. Possibilities can include some sort of infection (i.e. fungal infection, herpes or genital warts) or a skin condition (i.e. dermatitis).
You may want to obtain another opinion - either another urologist or dermatologist.
If all the tests continue to be non-revealing, a biopsy can be considered for a more definitive diagnosis.
Followup with your personal physician is
essentialEssential balance
Essential hypertension
Essential tremor.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b