The screening visit will take about 3 to 4 hours, and the study will take about 28 to 49 days. For 3 study days, participants will remain in the clinic for 12 hours, On Study Day 1, patients will be asked to not eat or drink anything besides water before coming to the clinic. Blood samples will be collected to measure prednisolone concentration as well as osteocalcin, a blood protein involved in bone formation. Blood tests will also allow a look at the surfaces of some white blood cells. Patients will receive a breakfast along with 20 mg of prednisone. Urine samples will be collected also. Then patients will be dismissed and asked to collect urine samples and return them on the following morning, Study Day 2. At that time there will additional blood collection. Patients will begin taking ritonavir between 7 and 28 days after the first dose of prednisone, at a dose of 200 mg (two 100 mg capsules) twice daily with food, to continue for 15 days. On the 4th and 14th days of their taking ritonavir, patients will be asked to visit the clinic in the morning for a blood to measure ritonavir in the blood and to take a single 20-mg dose of prednisone along with the ritonavir. Both medications will be taken with breakfast. Blood and urine levels of prednisolone will also be measured. On the 15th day of ritonavir dosing, there will be another blood collection for routine lab tests, as well as studies on blood cells and osteocalcin.