What can cause an increased risk of adverse reactions in elderly patients?

B is the correct answer because decreased renal function is a common physiological change in elderly patients that can significantly impact how medications are processed in the body. As individuals age, their kidneys often become less efficient at filtering waste products and excreting medications, which can lead to accumulating drug levels in the bloodstream. This accumulation raises the risk of adverse drug reactions, as the body may not eliminate medications quickly enough, leading to toxicity.

Increased metabolism typically does not occur in elderly patients; instead, metabolism often decreases with age, which can also affect drug clearance but is not the most direct factor for the increased risk of adverse reactions. Decreased muscle mass may have some implications for drug distribution but is less relevant than the effects of renal function. Increased hydration generally helps to improve renal function and can help manage medication clearance rather than causing an increased risk of adverse reactions.