Pharmacokinetics And Pharmacodynamics Practice Test

A 59-year-old female (weight 72 kg, height 162 cm) with rheumatoid arthritis is stable on methotrexate but develops mouth sores and fatigue after starting a new medication. Current medications: methotrexate 15 mg by mouth once weekly, folic acid 1 mg by mouth daily, pantoprazole 40 mg by mouth daily (started 3 weeks ago), naproxen 500 mg by mouth twice daily. Labs: serum creatinine 1.3 mg/dL (baseline 0.9 mg/dL), aspartate aminotransferase 45 U/L (high; normal <40), alanine aminotransferase 52 U/L (high; normal <40). Allergies: none. Therapeutic goal: control rheumatoid arthritis while preventing methotrexate toxicity. Which drug interaction is most likely affecting the patient's drug levels?

A 62-year-old female (weight 75 kg, height 166 cm) with chronic atrial fibrillation is treated with diltiazem and metoprolol and presents with dizziness and near-syncope. Current medications: diltiazem extended-release 240 mg by mouth daily, metoprolol tartrate 50 mg by mouth twice daily, apixaban 5 mg by mouth twice daily. Vitals: heart rate 42 beats/min, blood pressure 92/56 mmHg. Labs: serum creatinine 0.9 mg/dL, aspartate aminotransferase 20 U/L, alanine aminotransferase 18 U/L. Allergies: none. Therapeutic goal: adequate rate control without symptomatic bradycardia. Which pharmacodynamic effect is responsible for the patient's symptoms?