In elderly patients, renal function declines due to age-related changes in glomerular filtration rate and renal blood flow. This reduces **renal clearance** of drugs, leading to increased drug levels and risk of toxicity. While liver metabolism and gastric absorption also change with age, renal clearance is most significantly affected. Hypersensitivity is an immune response, not a pharmacokinetic change.

**✓ Correct: C. Renal clearance**