A 58-year-old man with uncontrolled hypertension (BP 180/110 mmHg) and no prior cardiac workup presents to a private clinic with acute chest pain. A cardiologist performs an ECG, which shows nonspecific ST changes. Without ordering troponin, echocardiography, or stress testing, and without referring the patient to a hospital, the cardiologist prescribes oral antihypertensives and sends the patient home. The patient suffers an acute myocardial infarction 6 hours later and develops cardiogenic shock. He sues the cardiologist for professional negligence. Which of the following best describes the legal standard by which the cardiologist's conduct will be judged?