After months of uncertainty, the Food and Drug Administration offered accelerated approval for an ultra-rare disease drug that had become a symbol of the fraught balancing act between upholding regulatory standards and accommodating desperate patients and their families.

The agency endorsed a treatment for Barth syndrome , a rare illness that afflicts about 150 people in the U.S. That decision comes after a protracted debate between the manufacturer, a small, privately held company called Stealth BioTherapeutics, and the FDA about how to generate enough of the right kind of data to prove the drug is effective in treating this tiny population of patients. The drug will be sold under the brand name Forzinity.

Over the past few years, the company endured a roller coaster as it attempted

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