A 72-year-old man with chronic lymphocytic leukemia is brought to the emergency department after his wife noticed he had been confused and increasingly difficult to rouse over the past day. He reports 3 days of worsening headache and fever. He has no known drug allergies. Temperature is 38.9°C, and neck stiffness is present on examination. Lumbar puncture reveals a CSF white blood cell count of 310 cells/μL with a mononuclear predominance, protein of 142 mg/dL, and glucose of 28 mg/dL (serum glucose 88 mg/dL). Gram stain of the CSF reveals small gram-positive rods. The treating physician initiates empiric ceftriaxone for bacterial meningitis.
Which of the following beta-lactam antibiotics is the drug of choice to add to this patient's empiric regimen to cover the most likely causative organism?