QT prolongation

A 72-year-old man with a history of heart failure and hypertension is admitted to the hospital with pneumonia. His medications include furosemide, ramipril, and digoxin. On day 3 of admission, he develops new-onset polymorphic ventricular tachycardia (Torsades de Pointes). An ECG shows a QTc interval of 580 ms. Which of the following is the MOST likely contributing factor to his arrhythmia?

A 72-year-old woman is admitted to the cardiology ward following an episode of syncope. Her past medical history includes hypertension, for which she takes hydrochlorothiazide, and osteoarthritis, managed with regular ibuprofen. An ECG reveals a prolonged QTc interval of 520 ms. She denies any family history of sudden cardiac death or prolonged QT syndrome. Her electrolytes are within normal limits except for a potassium level of 3.4 mmol/L (normal range 3.5-5.0 mmol/L). Which of the following is the MOST appropriate initial step in managing this patient?

A 78-year-old woman with a history of heart failure (reduced ejection fraction), type 2 diabetes, and recent hospitalisation for pneumonia presents to her GP with increasing fatigue and a single episode of dizziness leading to a fall without loss of consciousness. Her current medications are Digoxin 125 mcg daily, Furosemide 40 mg daily, Ramipril 5 mg daily, Metformin 500 mg BD, and Amoxicillin 500 mg TDS (started 5 days ago for pneumonia). An ECG in clinic shows sinus rhythm, rate 70 bpm, and a QTc of 530 ms. An ECG from 6 months prior had a QTc of 440 ms. Recent laboratory tests show Na 138 mmol/L, K 4.2 mmol/L, Mg 0.9 mmol/L, Ca 2.3 mmol/L, Creatinine 90 µmol/L, and Digoxin level 1.1 ng/mL. Considering the clinical context and investigations, which of the following is the most likely significant contributor to the observed QTc prolongation?

A 35-year-old patient with schizophrenia is on risperidone and develops QT prolongation on ECG. What is the next step?