What Causes Burning Sensation In The Penis?

Hi ,

I have mild buring in my penis in the urethra region since last one month. Its a constant mild burning throughout the day. I have checked with urologist who ruled out STD and UTI . I have done a urine analysis which came back negative.I donot have lesion, cuts, rashes, warts , or any discharge. I donot have painful urination either. Its just this constant mild burning through out the day. This is reducing my sexual interest too. The urologist has asked me to do a penile doppler, to check the functioning of penis. I have a few questions pls:

-What could be the cause of my problem since the doc has ruled out STD and UTI?

-Will Penile doppler help in finding the cause?

-Can i do a penile doppler without injecting Pge2? Cant penile doppler be done with manual erection?

- Could this be because of some scar tissues or urethral stircture, not sure how it could develop suddenly?

I am very tensed because of this problem. Pls help me