A newly diagnosed patient is prescribed lithium. Which information from the patient's history indicates that monitoring of serum concentrations of the drug will be challenging and critical?

Rationale: A. Arthritis does not typically affect lithium metabolism or clearance. B. Heart failure can impair renal function and fluid balance, increasing the risk of lithium toxicity and making serum concentration monitoring critical. C. Psoriasis does not interfere with lithium pharmacokinetics. D. Epilepsy is not directly related to lithium metabolism, though caution is warranted if the patient is on other medications.