A 38-year-old woman presents with jaundice, right upper quadrant pain, and fever. Laboratory tests show total bilirubin 6.5 mg/dL (predominantly conjugated), alkaline phosphatase 420 U/L, GGT 380 U/L, and normal transaminases. Ultrasound abdomen reveals dilated intrahepatic and extrahepatic bile ducts with a 1.2 cm stone impacted in the common bile duct. ERCP is contraindicated due to anatomical variation. What is the drug of choice for acute management of cholangitis in this patient?