Doctors diagnosed him with kidney failure. A Russian journalist said doctors at the hospitaltold him that they "suspect poisoning." But Kara-Murza's father told a newspaper, "The deputy head doctor of the hospital came out and told us that everything was fine with his heart, his lungs, his stomach, etc. It all had to do with his kidneys. It could have been spoiled yogurt or something else."
My bet is on "something else."