Which patient has the greatest predisposition to an adverse reaction when administering medications?

The 75-year-old woman with cystitis has the greatest predisposition to an adverse reaction when administering medications primarily due to age-related physiological changes. As individuals age, their bodies typically undergo various alterations in pharmacokinetics and pharmacodynamics, which can affect drug metabolism and excretion processes.

In older adults, organ systems such as the liver and kidneys may not function as efficiently, leading to impaired clearance of medications from the body. This can heighten the risk of drug accumulation and, consequently, adverse drug reactions. Additionally, older populations often have multiple comorbidities and may be taking several medications concurrently, increasing the likelihood of drug-drug interactions and side effects.

In comparison, while the other patients may also be at risk for adverse reactions, they do not have the same level of vulnerability associated with advanced age and its related physiological changes. For instance, the 9-year-old boy's physiology is generally more adaptable, the 50-year-old man's health status may not indicate significant risk factors, and while the 30-year-old man with kidney disease does have a specific concern, his age suggests a potentially more robust system in processing medications compared to the older woman.