Unstable angina is defined as?

Unstable angina is characterized by chest pain that occurs unexpectedly and typically at rest, rather than being triggered by physical exertion or emotional stress. This type of angina indicates a significant change in the pattern of pain and is often associated with an increased risk of heart attack. The pain may be more intense, frequent, or prolonged than stable angina, which is generally predictable and relieved by rest or nitroglycerin.

In contrast, the other options describe conditions that do not align with the definition of unstable angina. For instance, chest pain that resolves quickly with rest refers to stable angina, not unstable. Constant chest pain that's unrelated to any activity may suggest a different pathology, possibly indicating a myocardial infarction or other serious conditions rather than unstable angina. Angina triggered by emotional stress is also more aligned with stable angina, as it typically occurs due to increased demand on the heart during stressful situations and is not indicative of the rest-related pain seen in unstable angina.