Possible causes can include infection or a dermatitis. You need to be tested for STDs and urethritis (i.e caused by gonorrhea and chlamydia) this can be done via a urine test or urethral swab.
Another possible cause would be anatomical abnormalities, such as a urethral stricture or polyp. A cystoscopy can be considered if an infection has been ruled out.
I would suggest a urology or dermatology referral for further evaluation.
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