A Healthy 25-Year-Old Woman Donates a Kidney to Her Identical

A healthy 25-year-old woman donates a kidney to her identical twin,who has chronic renal failure.Her serum creatinine concentration before removal of the kidney is 1.0 mg/dL.After removal of her kidney,her serum creatinine concentration increases to 2.0 mg/dL.One month later her serum creatinine concentration has decreased to 1.5 mg/dL.Which of the following accounts for the fact that her serum creatinine concentration fell from 2.0 to 1.5 mg/dL?

A) Decreased production of creatinine by skeletal muscle
B) Enhanced secretion of creatinine by the proximal tubule
C) Increase in the GFR of each of the remaining nephrons
D) Expansion of the extracellular fluid (ECF) volume