A 38-year-old woman with severe hepatic cirrhosis (Child-Pugh Class C) is started on metoprolol 50 mg twice daily for portal hypertension. Her plasma albumin is 2.8 g/dL (normal 3.5–5.0), and INR is 2.1. After 3 days, she develops severe bradycardia (heart rate 42/min) and hypotension (BP 88/54 mmHg). Serum metoprolol concentration is 450 ng/mL (therapeutic range 50–200 ng/mL). Which combination of pharmacokinetic factors best explains this toxicity?