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

The client with the greatest predisposition to an adverse reaction when administering medications is the 75-year-old client with impaired kidney function. This is due to the fact that aging is associated with changes in pharmacokinetics, including altered absorption, distribution, metabolism, and excretion of medications.

In this specific case, impaired kidney function significantly impacts the body's ability to eliminate drugs efficiently. Medications that are primarily cleared through the kidneys may accumulate to toxic levels, increasing the likelihood of adverse reactions. This age group is particularly sensitive to medications, and the presence of impaired renal function further compounds this risk, as both age and kidney health influence how the body processes drugs.

In contrast, the younger clients listed, such as the 30-year-old with no chronic illnesses and the 9-year-old with a recent cold, generally have a better physiological reserve and metabolic capacity to handle medications. The 50-year-old client with seasonal allergies may have some predisposition to specific medications related to allergy treatment, but it does not compare to the systemic risks posed by renal impairment in an older adult.