A 52-year-old man with type 2 diabetes mellitus presents to a private clinic with complaints of persistent headache and blurred vision for 3 days. The physician, without performing any fundoscopic examination or blood pressure measurement, prescribes oral analgesics and asks the patient to return in a week. The patient develops sudden vision loss the next day and is diagnosed with hypertensive retinopathy and acute angle-closure glaucoma at a tertiary centre. The patient files a suit against the physician for negligence. Which element of professional negligence is MOST clearly established in this case?