I would certainly consider a referral to a urologist. If the normal tests for STDs are negative, including gonorrhea, chlamydia and UTIs, then you may want to investigate the prostate. A thorough rectal exam would be a good start, as well as sending off any prostatic fluid for analysis and culture.
I'm not sure a penile ultrasound would be helpful with the irritation. Sometimes anatomical abnormalities such as urethral polyps or strictures can cause irritation and can be diagnosed via a cystoscopy.
Going back to the prostate, a transrectal ultrasound can be considered to image the prostate, looking for any abscesses which can cause chronic infection.
You may want to discuss these options with your personal physician or urologist.
Followup with your personal physician is essential.
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