A 21-year-old male presents with fatigue, poor appetite, nausea, and right upper quadrant discomfort for 4 days. No history of hepatitis. Physical exam reveals icteric sclera and a tender, mid-firm liver palpable 2 cm below the costal margin. Suspected hepatitis with ALT 2460 U/L confirms acute viral hepatitis. The most critical treatment measure at this stage is: