A nurse is caring for a client receiving febuxostat for long-term treatment of gout. Which of the following adverse effects should the nurse closely monitor for?

The correct answer is D: Renal impairment. Febuxostat is a xanthine oxidase inhibitor used to treat gout by reducing uric acid levels. The nurse should monitor for renal impairment as febuxostat can cause kidney damage. This is crucial for long-term treatment, as renal function is essential for excreting uric acid. Hypertension (A) is not a common adverse effect of febuxostat. Malignant hyperthermia (B) is a rare, life-threatening reaction to certain medications, typically associated with anesthesia. Steven's Johnson Syndrome (C) is a severe skin reaction, not typically associated with febuxostat.