A 58-year-old man with known compensated liver cirrhosis due to alcohol presents with a 2-day history of increasing confusion, drowsiness, and a flapping tremor (asterixis) of his outstretched hands. His partner reports constipation for the last 4 days. His observations are stable. Blood tests show: Na+ 136 mmol/L, K+ 3.9 mmol/L, Urea 7.5 mmol/L, Creatinine 88 µmol/L, Bilirubin 40 µmol/L, ALT 55 U/L, Ammonia levels are elevated. What is the most appropriate initial pharmacological treatment to manage his current presentation?