The main characteristic of unstable angina is:

Unstable angina is primarily characterized by episodes of chest pain that occur unpredictably, often at rest, and can last longer than 30 minutes. This condition signifies a critical decrease in the blood flow to the heart due to the narrowing or blockage of the coronary arteries. Pain associated with unstable angina is often more intense and distressing compared to stable angina and does not necessarily relate to physical activity or stress levels. The pain can also occur during periods of inactivity, which distinguishes it from stable angina, where pain is typically triggered by exertion and resolves with rest.

The other options describe different aspects of chest pain that are not typical of unstable angina. Predictable pain patterns lasting less than 5 minutes and intermittent episodes resolving with rest suggest more stable patterns of angina, which indicate a lower risk for heart complications. Random episodes of severe chest pain during activity may point towards other angina classifications but do not capture the defining characteristics of unstable angina, particularly the sustained and unpredictable nature of the pain.