The nurse should teach the patient to report which of the following to the health care provider immediately if experienced after taking nitroglycerin?

A) Flushing of the face:
Flushing is a common and usually harmless side effect of nitroglycerin, caused by its vasodilation properties. It often resolves quickly and is not typically a cause for concern. While it can be uncomfortable, it is not a sign of a serious problem that would require immediate medical attention.

B) Dizziness upon standing:
Dizziness upon standing (orthostatic hypotension) is a known side effect of nitroglycerin due to its blood pressure-lowering effects. It typically occurs when the patient rises too quickly from a sitting or lying position. While it can be uncomfortable and may cause the patient to feel lightheaded, it is usually not an emergency and can often be managed by rising slowly. If dizziness persists or is severe, the healthcare provider should be notified, but it is not an immediate emergency.

C) Persistent chest pain that lasts longer than 15 minutes:
Chest pain lasting longer than 15 minutes could indicate a myocardial infarction (heart attack) rather than stable angina, even if nitroglycerin has been administered. Chest pain that is not relieved by nitroglycerin or persists for more than a few minutes is an emergency situation and should be reported immediately to the healthcare provider or emergency services. 

D) Mild headache:
Headache is a common and benign side effect of nitroglycerin, caused by its vasodilation effect. It is usually mild and transient and does not require immediate medical attention. In most cases, the headache improves over time or with the use of over-the-counter pain relievers, and it is not a reason to contact the healthcare provider unless it becomes severe or persistent.