GFR and Renal Clearance

A 38-year-old woman with systemic lupus erythematosus (SLE) is referred to the nephrology clinic for evaluation of lupus nephritis. Her serum creatinine is 1.8 mg/dL, blood urea nitrogen is 35 mg/dL, and serum sodium is 136 mEq/L. A 24-hour urine collection shows a total protein excretion of 4.2 g/day and urine creatinine of 800 mg/day. Her body weight is 60 kg. Using the Cockcroft-Gault equation (assuming age 38, female, non-obese), her estimated GFR is approximately 32 mL/min/1.73 m². The nephrologist orders a renal biopsy, which confirms Class IV lupus nephritis with 60% glomerular involvement. Before initiating immunosuppressive therapy, the team needs to assess baseline renal function. Which of the following statements about renal clearance measurement in this patient is most accurate?

A 52-year-old man with chronic kidney disease stage 3b (eGFR 35 mL/min/1.73 m²) undergoes simultaneous measurement of urea clearance and inulin clearance. Which finding would best distinguish urea clearance from inulin clearance as a measure of true GFR?

A 62-year-old man with chronic kidney disease (CKD stage 3b) has a measured 24-hour urine creatinine clearance of 48 mL/min and a serum creatinine of 1.8 mg/dL. Which finding best distinguishes his measured creatinine clearance from his true glomerular filtration rate?

Renal clearance of a substance is defined as the volume of plasma that is completely cleared of that substance per unit time. Which of the following relationships correctly describes the renal clearance formula?

A 38-year-old woman with systemic lupus erythematosus is being monitored for lupus nephritis. Her serum creatinine is stable at 1.2 mg/dL, but you suspect early glomerular dysfunction not yet reflected in creatinine. Which investigation is most sensitive for detecting early decline in glomerular filtration?