
Okay, you did the right thing in having the determination to do something about this
You need to be clear about the circumstances that you currently have with your penis, that, they consist of two entirely separate issues: i.) what you see in the photo; and, ii.) what you feel, that is, the mild discomfort at the root of the penis shaft. These issues are extremely unlikely to be connected.
In detail, when you state:
" I have observed a dark discoloration on the corona of the head of penis. The discoloration is not just in a one site, but is symmetrically present all through the round border of the corona of the penis "
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… this in the main is likely to be the normal make up of your penis and is simply how it has naturally developed - many men have this, from barely noticeable to being very clearly present across the entirety of the head of the penis. This is normal and is not a problem in any way.
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However, I am concerned with what I see right in the centre of the back of the head of your penis and the accompanying red marking right at the top of the shaft of your penis immediately below the head.
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What you need to do, so that I can further comment is to take a photo of the same area as in your existing photo - take it a little closer, but you need to take the photo in bright daylight - such as to the side of a window, indoors where you cannot be observed from outside …