A plausible medical background? Other than low salt diet, leeching and purging, little treatment was available for high blood pressure at that time. And kidney failure is a recognized complication of uncontrolled hypertension. However, his high blood pressure could also have been the consequence of progressive kidney disease, with genetic causes such as polycystic kidney disease an alternative diagnosis - in the absence of confirmation of diagnoses in father and son by histology. A non-genetic cause in father and son could have been post-infective glomerular disease (such as Bright's disease) - Bulgakov gave up a career as a doctor because of an illness. This could have caused the onset of kidney disease, which, depending on the cause, may take up to several decades to progress to kidney failure.
This medical background raises the question: did long-term illness influence Bulgakov's literary creativity? There are two obvious potential effects: knowledge, especially as a doctor, of premature death giving the drive to create as much as possible; and effect of the disease itself on thoughts and imaginings. Of note, in the late stage, kidney disease is well-known typically to cause slowing of mental processes. There are apocryphal links between Tolstoi's novel 'War and Peace'. However I experienced a real example as a junior doctor in London when a women was admitted to hospital in end-stage kidney disease. She was obtunded by her illness, however within a day or so of starting haemodialysis had recovered her previous passion for reading and was able to make accelerating progress through the book, obvious during successive ward rounds.
Modern science has revealed that cytokines, pathological factors involved in a wide range of medical problems, including inflammatory kidney disease, have important effects to influence mood and other aspects of brain activity, to the extent that trials are in prospect targeting cytokines as a way to treat depression. And of note Bulgakov wrote The Master and Margarita during the latter part of his last illness. Many researchers have been interested in the link between creativity and depression - and several obvious reasons for Bulgakov to be depressed in Moscow at that time, independent of his own health. For individual creative people, very challenging to differentiate between direct or indirect cause, and coincidence, both for creativity and its loss...