Antimicrobials USMLE Step 1 Practice Question

A 72-year-old man with a 20-pack-year smoking history and moderate-to-severe COPD presents with a 3-day history of worsening dyspnea, productive cough with purulent sputum, and low-grade fever. Sputum cultures from a prior hospitalization grew Pseudomonas aeruginosa. He is started on cefepime 2 g IV every 8 hours for presumed Pseudomonas pneumonia. Pertinent medical history includes hypertension, type 2 diabetes mellitus, and chronic kidney disease stage 3 (baseline creatinine 2.1 mg/dL). On hospital day 2, nursing reports acute onset confusion, myoclonic jerking of the arms, and visual hallucinations. Vital signs are temperature 37.1°C, blood pressure 140/84 mmHg, heart rate 86 bpm, and oxygen saturation 95% on 2L nasal cannula. Serum creatinine is now 2.4 mg/dL (up from 2.1 mg/dL); electrolytes including sodium 138 mEq/L and glucose 124 mg/dL are within normal limits. Electroencephalography shows generalized slowing with high-amplitude slow waves. Non-contrast CT head is unremarkable. Which of the following is the most likely explanation for this patient's neuropsychiatric symptoms?