A client with a long-standing diagnosis of chronic kidney disease has been experiencing increasing fatigue, lethargy, and activity intolerance in recent weeks. His care team has established that his GFR remains at a low, but stable, level. Which of the following assessments is most likely to inform a differential diagnosis?

A) Blood work for white cells and differential
B) Cystoscopy and ureteroscopy
C) Assessment of pancreatic exocrine and endocrine function
D) Blood work for hemoglobin, red blood cells, and hematocrit