I don't buy it. I've had health problems from birth, myself. My medical record is quite extensive and admittedly, I don't remember every single thing. However, my parents do. Whenever I see a new Dr or have medical procedures done, I always bring my Mother with me so we can give as complete a history as we possibly can. Even if Nika doesn't remember everything, somebody has the info and he should have access to it.
Neither I, nor my mother, remember Every Single Thing, and I know quite a bit more than she does in the past few years (and gd knows it's a quite extensive list of things) but I do keep copies of all my records from birth to consult to, save for a few that were not aqiured for some reason, were lost, ect. Anyone with any medical condition should know this drill- you are, after your GP, the central information keeper of all your care being managed by specialists, both now and in the past.
I might not remember the details (all tests performed, especially at a young age, or specific results like "130mg/dl on x back in 03/01/2001" or whatever), but pretty much anyone, especially an adult, will know their diagnosis, the cause(s), treatment, so on.
This is fairly normal.
I've had quite a few people, disturbingly, that I've encountered and it seemed very much like they were 'faking blind'-- one of the most obvious red flags for me is when someone said something along the lines of "I'm going blind, I have a family history, all the men in my family went blind by x age" but they were not able to identify the disease name and said that although they were born legally blind (how can you be going blind when you're already blind? You can lose more sight...) they are not seeing an ophthalmologist, do not know their diagnosis, so on. HUGE red flags all around.
On the flip side, I do not take having an extensive knowledge of one's disease to be a 'flag', especially with disabling conditions. I think many of us arm ourselves with up to date information and can scare off pros with our 'unacceptable' levels of understanding.. unfortunately, the relatively clueless patient, or worse, the one who hit WebMD is the standard...
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Either way, I don't think it's OK to accuse people unless you're absolutely willing to call them out as a liar.
I had a neuro-ophthalmologist tell me that I could see more than I said I said I could see, and this person made my parents think the same thing (that I was 'faking/imagining' my vision being as bad as it was... despite having a diagnosis proven by several objective tests...) and it was a very distressing time where I felt like I was being accused by a lot of people when I knew I was telling the truth. I didn't see her again and saw another doctor who in the first appointment told me she believed me which relieved a lot of the stress and anger, but the time in between that was not great all the same, and people still kept pulling the "but that doctor said.." card some time later.
Mine was a very short period of time, but some people might go years being not believed, perhaps even a lifetime... and I don't want to, unless it's absolutely obvious that the person is lying, not believe someone when they say something is wrong, for the chance that they might be telling the truth.