Joshua M. Sharfstein, the second-in-command at the Food and Drug Administration, is leaving that post after less than two years to become Maryland's top public health official, Gov. Martin O'Malley said Tuesday. The offer from O'Malley came two weeks ago after John M. Colmers told the governor that he intended to step down after four years as Maryland's secretary of health and mental hygiene. "I was committed to the FDA. But opportunities like this don't come along very often. It's a really great job at a really important moment in time, and with terrific leadership in the state," Sharfstein said. The new job, which Sharfstein will start next week, touches a broad array of public health issues, including infant mortality, HIV/AIDS, substance abuse, mental health and implementation of healthcare reform.