An elderly, critically ill patient who was hospitalized underwent gastric surgery 1 week ago. the caregiver reports that the patient has seemed confused for the past 2 to 3 days. which drug would most likely have caused this symptom in the patient?

The drug is not as important as the symptom here. It is clear that the patient is suffering from an irregular heart rate, due to the confusion after the gastric surgery. There are many drugs that can cause irregular heart rates, like caffeine, tobacco, alcohol, and even cold and cough medications.