Drug Prescribing and Monitoring — Drug interactions

A 66-year-old man presents to the emergency department with 10 days of aching and weakness in both thighs and shoulders, difficulty rising from a chair, fatigue, and dark urine. He had a myocardial infarction 3 years ago and has hypertension, asthma, and erectile dysfunction. Metoprolol was stopped last year because of bronchospasm. His medications are ASA 81 mg daily, ramipril 10 mg daily, atorvastatin 80 mg daily for 3 years, tadalafil 5 mg daily, and salbutamol as needed. Diltiazem CD 180 mg daily was started 3 weeks ago for exertional chest pain. On examination, his proximal muscles are tender with 4/5 strength in the hip flexors and deltoids. Laboratory results: creatine kinase 3800 U/L (less than 200), creatinine 132 µmol/L (60-110; baseline 88 µmol/L three months ago), potassium 4.9 mmol/L (3.5-5.0), AST 160 U/L (less than 40), and TSH normal. Urine dipstick is positive for blood, but microscopy shows no red blood cells.