Based on the huge numbers of prescriptions written by a Miami psychiatrist, Sen. Charles Grassley, R-Iowa, is continuing to pressure federal officials to investigate why some doctors write stunning numbers of scripts for tax-funded Medicare and Medicaid programs. In his latest volley, a letter sent Wednesday to Kathleen Sebelius, secretary for the U.S. Department of Health and Human Services, Grassley demanded exact answers to three pointed questions about what her department is doing to address the problem. “The federal government has an obligation to figure out what’s going on here,” Grassley said in a statement e-mailed to The Miami Herald Wednesday. “The taxpayers are footing the bill, and Medicare and Medicaid are already strained to the limit. These programs can’t spare a dollar for prescription drugs that aren’t properly prescribed.