 After collection of the baseline blood sample and a separate sample for tissue banking optional, 50 mg Inulinkg will be administered by bolus IV, followed by a sustaining dose of 18.8 mgsquare areamin. Approximately 30 minutes later, 35 μCuries of 125I-iothalamate will be injected subcutaneously to allow for a 60 minute equilibrium time. After a further 30 minutes, 5 ml of Iohexol Omnipaque 300, 300mgmL of organic iodine and 10 uLkg of Gd-DTPA will be administered via a bolus IV injection through the same IV line as inulin. Iohexol will be given first over 1 minute, followed by saline flush of the intravenous line, followed by Gd-DTPA over one minute. Subjects with an eGFR 30 mLmin1.73 m2, as determined from their medical record or the serum creatinine measurement from the health screening, will not receive Gd-DTPA. The syringes used to administer the iohexol and Gd-DTPA will be weighed to the nearest tenth gram on the same scale before and after injection. Nurses at the CTRC will monitor the participant at the time of the administration and throughout the study. 