1. A 40-year-old woman presents to the emergency department because of hematuria. Laboratory analyses show significant proteinuria, bacteria and white cells in the urine, and a blood urea nitrogen (BUN) of 40 mg/dL with a creatinine of 4.0 mg/dL. Ultrasonography reveals enlarged kidneys, and she is given a provisional diagnosis of polycystic renal disease. Which of the following is likely to be decreased in this patient?