Urinary sepsis: empirical intravenous antibiotics including gentamicin in an obese patient with renal impairment

Patient B-217, a 72-year-old man, presents with a 2-day history of fever, dysuria and left flank pain. He has also felt light-headed and reports decreased urine output. He attends the emergency department with suspected sepsis. On examination: temperature 38.9°C, heart rate 118/min, blood pressure 88/56 mmHg, respiratory rate 26/min, oxygen saturation 95% on room air. He is drowsy but rousable. There is left renal angle tenderness. Urine dipstick shows leucocytes and nitrites. Blood results: haemoglobin 128 g/L, white cell count 18.2 × 10^9/L, platelets 180 × 10^9/L, serum creatinine 1.3 mg/dL (eGFR 55 mL/min/1.73 m^2), urea 12.0 mmol/L, sodium 135 mmol/L, potassium 4.1 mmol/L. Blood cultures have been taken. His weight is 110 kg and height 175 cm. He has type 2 diabetes mellitus (on metformin 1 g twice daily) and hypertension (on ramipril 10 mg daily). He has no known drug allergies. Task: prescribe the initial intravenous antimicrobial regimen for this patient, including a once-daily gentamicin dose calculated using the correct body weight, and specify the required level monitoring.