What distinguishes unstable angina from stable angina?

Unstable angina is characterized by its unpredictable nature and can occur at rest, even without any physical exertion. Unlike stable angina, which typically manifests during exertion and is relieved by rest or nitroglycerin, unstable angina presents a more serious condition that may signal an impending heart attack. This type of angina often lasts longer than stable angina and does not resolve with rest, indicating that the heart muscle is not receiving enough blood flow due to obstruction of the coronary arteries.

In the context of the other choices, stable angina is indeed predictable and typically occurs during activity, thus excluding the notion that unstable angina is solely activity-induced. While stress can contribute to angina symptoms, it is not the only factor that triggers unstable angina, making the assertion that it is solely caused by stress incorrect. Moreover, stable angina episodes generally last only a few minutes and are distinguishable from the unpredictable and more severe nature of unstable angina. This understanding is crucial for differentiating between the two conditions and managing patient care effectively.