Q. A 65-year-old woman is brought to the emergency department because of nausea, diarrhea, and vomiting for three days duration. Her daughter mentions that her mother acted strangely during last week's family gathering. The patient's medical history is significant for heart failure, diabetes, atrial fibrillation, and Alzheimer's dementia. Her medication list is unavailable. The patient's vital signs are normal except for frequent skipped beats. The patient's daughter is convinced her mom is "over-drugged". She provides a list of medications and says the most recent one she was started on is digoxin. Which of the following best describes the mechanism of action of digoxin?