Amid a severe kidney-donor shortage, an idea long considered anathema in the medical community is gaining new currency: payments for people willing to give up a kidney. One of the most outspoken voices on the topic isn't a free-market libertarian, but a prominent transplant surgeon named Arthur Matas. Dr. Matas, 59 years old, is a Canadian-born physician best known for his research at the University of Minnesota. Lately, he's been traveling the country trying to make the case that barring kidney sales is tantamount to sentencing some patients to death. "There's one clear argument for sales," Dr. Matas told a...