Hi Doctor I have been active on sex till 6 months ago however i am not active now since i did notice small blister kind of lumps on my Penis. I did check with couple of doctors and also underwent STD test (Blood test) which came down as negative. I do masturbate quite frequently so the tip of my Penis use to be bit dark than the rest of the shaft and in the resent times i noticed that the texture of that dark part is different from the other regions. When i checked with the doctors i was advised that i was bcos of mastubation. I do not feel any pain or discomfort on that part. Please advise if that could be something serious. Thanks doctor.