On 27 May 2015 Alec Luhn and Luke Harding wrote in The Guardian:

'Doctors initially thought [Vladimir Kara-Murza, Jr.,] could have been poisoned, Vadim Prokhorov, a lawyer for the RPR Parnas opposition party, told the newspaper Kommersant. [...] Doctors later diagnosed Kara-Murza with kidney failure, his father told RBC 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,” he said. “It could have been spoiled yogurt or something else.” Although Kara-Murza’s father ruled out deliberate poisoning, colleagues had expressed doubts about the sudden illness, which comes after the killing of Nemtsov and reported pressure on opposition activists. Open Russia project coordinator Maria Baronova told the Guardian his illness was suspicious and said “various activities surrounding public people from Open Russia look strange”, but declined to elaborate. Opposition journalist Alexander Ryklin wrote on Facebook on Wednesday that he had just spoken with Kara-Murza and that the doctors “suspect poisoning”.'Sources and further reading:

Alec Luhn and Luke Harding, 'Russian opposition activist Vladimir Kara-Murza in hospital after falling ill', The Guardian, 27 May 2015
Will Wright, 'Is Vladimir Kara-Murza's Hospitalization a Fluke or Part of a Violent Pattern?', Global Voices Online, 1 June 2015
Neil Buckley, Nemtsov ally in critical condition in Moscow', The Financial Times, 29 May 2015