What Happens If Istinja’ Water Enter the Urethra?
Question

I read on Islamic websites a number of Fatwas which say that if Istinja’ water enters the penis then comes out of it, it is regarded as impure (Najis) and invalidates Wudu. This is what is making me very anxious, because when I have a bath or clean myself after relieving myself (Istinja’), I noticed that some water enters the opening of the penis and remains inside it, then it comes out of it. 
There is no doubt about that; rather it is something certain, one million per cent. I asked a doctor and he told me that this water does not enter the urethra. This means that the water only enters the opening of the penis, but it does not enter the urethra itself. I have also noticed that there is always some moisture in the opening of the penis, at times other than cleaning myself after relieving myself or taking a bath. After lengthy research and asking a doctor, I came to know that the opening of the penis on the inside has a mucous membrane lining which produces moisture to protect this area, like the moisture inside the mouth. 
This moisture does not worry me at all, because it does not come out of the opening of the penis. But the problem is the fact that this moisture is present in the urethra, i.e., in the place of impurity (Najasah), and when having a bath or cleaning myself after relieving myself, some of the water enters the opening of the penis, and thus comes into contact with this moisture.
My question is twofold:
Firstly, what is the ruling on the water that enters the opening of the penis when having a bath or cleaning myself after relieving myself – does it become impure as a result of coming into contact with the moisture that is present inside the opening of the penis, through which urine has passed? 
Secondly, can we say that the water that enters the opening of the penis then comes out of it does not invalidate Wudu so long as it does not go all the way back to the urethra itself, and that its entry is limited only to the opening of the penis? In other words, is what matters in invalidating Wudu the urethra or the opening of the penis?
Praise be to Allah.What happens if Istinja’ water enters the urethra?
If someone causes water, oil or anything else to enter the urethra then it comes out, it is impure and invalidates Wudu.
Ibn Qudamah (may Allah have mercy on him) said:
“If he drops oil into his urethra, then it comes out again, it invalidates Wudu, because it came out of the passage, which cannot be free of some impure wetness that accompanies it. Therefore Wudu is invalidated thereby, just as when only impurity comes out." (Al-Mughni 1/125)
This applies if the water reaches the inside, in the sense that when it comes out it is regarded as coming out from the passage. But if it only reaches the opening of the penis , which is regarded as pure, and it does not go any deeper, then it has no impact.
What you have mentioned is no more than intrusive thoughts (Waswasah ), because it is difficult for water to enter inside the penis, unless the individual deliberately makes an effort to do that.
If we assume that it happened, then what you must do is to relieve yourself after doing Ghusl, and clean yourself (Istinja’ ) by washing the tip of the penis without opening it or examining it, and you should not do anything other than that, otherwise you will be opening the door to intrusive thoughts.
Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) said in Majmu` Al-Fatawa (21/106): 
“Examining the penis and squeezing out the last drop of urine, and the like, is all an innovation (Bid`ah) that is not required or recommended according to the leading scholars of the Muslims. Moreover, by the same token, shaking the penis is also an innovation according to the correct view, and it was not prescribed by the Messenger of Allah (blessings and peace of Allah be upon him).
Similarly, expressing the urine is an innovation that was not prescribed by the Messenger of Allah (blessings and peace of Allah be upon him). 
The Hadith which speaks of that is inauthentic and has no basis. The urine comes out naturally, and when one has finished it stops naturally. As it was said, it is like the udder, if you leave it alone it will dry up, but if you squeeze it, milk will flow.
Every time the individual opens the opening of his penis, something may come out, but if he leaves it alone nothing will come out. He may imagine that something has come out, but these are intrusive thoughts.
The one who experiences that may feel some coolness at the tip of the penis and think that something has come out of it, when nothing has come out.
Urine may be retained at the top of the urethra and not drip, but if he squeezes the penis or the opening with a rock, finger or anything else, some moisture will come out; this is also an innovation. Also, that retained urine does not need to be forced out, according to scholarly consensus, either with a rock or a finger or anything else. Rather every time he forces it out, more will come, for it is dripping all the time.
Cleaning oneself with stones is sufficient and there is no need to wash the penis with water. It is recommended for the one who does use water to sprinkle water on his private part, then if he feels any moisture, he can say: it is from that water.”
Is the water that remains on the penis after purifying it pure?
Water that remains on the penis after purifying it is deemed to be pure, because water that is separate from the impurity after cleaning it is pure.
Is it recommended to sprinkle some water on your underwear after cleaning yourself?
After you clean yourself (Istinja’ ) you should sprinkle some water on your underwear , so that if you notice any moisture after that, you can tell yourself that it is the water that you sprinkled in your underwear.
Ibn Majah (464) narrated that Jabir (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) did Wudu then sprinkled water on his private part. (Classed as authentic by Al-Albani in Sahih Ibn Majah)
Ibn Qudamah (may Allah have mercy on him) said: “It is recommended to sprinkle water on the private parts and trousers, in order to dispel intrusive thoughts.”
Hanbal (may Allah have mercy on him) said: “I asked Ahmad: I do Wudu after I cleanse myself, and I feel that I have invalidated my Wudu. He said: If you want to do Wudu, then in order to put your mind at rest, take a handful of water and sprinkle it over your private part, and do not pay attention to it [the intrusive thought], then it will go away, in Sha Allah." (Al-Mughni 1/115)
It says in Al-Mawsu`ah Al-Fiqhiyyah (4/125): “The Hanafis, Shafi`is and Hanbalis stated that when a person has finished cleaning himself with water (Istinja’ ) it is recommended for him to sprinkle his private part or his trousers with some water, so as to ward off intrusive thoughts, so that if he develops any doubts he will interpret that wetness as being from the sprinkling of that water, unless he is certain otherwise.”
What appears to us to be the case from your questions is that you are affected by intrusive thoughts (Waswas) ; may Allah Most Generous heal you of it by His grace. So distract yourself from these intrusive thoughts as much as you can, and seek refuge with Allah from them. We advise you to consult a specialist doctor, for these intrusive thoughts are a sickness, like any other sickness. So if you combine medicinal treatment with the spiritual treatment of seeking refuge with Allah and Ruqyah, and behavioural therapy with a therapist who understands that, that will be good for you, and I hope that you will recover from it, in sha Allah.
And Allah knows best.