US regulators have approved the most expensive medicine ever, for a disorder that destroys a baby’s muscle control and kills nearly all of those with the most common variant of the disease within a couple years. The treatment is priced at $2.125 million. Out-of-pocket costs for patients will vary based on insurance coverage. The gene therapy, developed by Novartis, treats a rare condition called spinal muscular atrophy, which strikes about 400 babies born in the US each year. The Food and Drug Administration on Friday approved the drug, called Zolgensma, for all children under age 2 who are confirmed by...