Which antiviral is contraindicated in patients with severe renal impairment?

Acyclovir is contraindicated in patients with severe renal impairment because its clearance is heavily dependent on renal function. Acyclovir is primarily eliminated by the kidneys through glomerular filtration and tubular secretion. In patients with impaired renal function, the decreased clearance can lead to increased serum levels of the drug. This accumulation increases the risk of adverse effects, particularly neurologic toxicity, such as confusion, tremors, and even seizures.

In contrast, the other antiviral agents listed have different pharmacokinetic profiles or mechanisms of action that do not pose the same level of risk in patients with renal impairment. For instance, Ribavirin, while also requiring dose adjustments in the context of renal impairment, is primarily eliminated through the liver. Tenofovir can lead to nephrotoxicity, but its association with severe renal impairment does not have the same acute risk profile as Acyclovir. Sofosbuvir is metabolized primarily by the liver and therefore does not require similar caution with respect to renal function.

This understanding emphasizes the importance of assessing renal function when prescribing antiviral medications to optimize safety and efficacy in treatment.