Which of the following is an example of a life-threatening sudden illness?

A heart attack is a critical example of a life-threatening sudden illness because it involves the heart's inability to receive adequate blood supply, which can lead to severe complications, including death, if not addressed immediately. The sudden onset of symptoms, such as chest pain, shortness of breath, or discomfort in other areas of the upper body, requires immediate medical attention.

In contrast, the other conditions listed—like a cold, a sprained ankle, and a headache—do not typically pose an immediate threat to life. A cold is usually a mild respiratory illness that resolves on its own, a sprained ankle, while painful, is manageable with first aid and does not endanger life, and a headache may stem from various benign causes that are generally not life-threatening. Therefore, recognizing the signs and urgency of situations like a heart attack is crucial for effective intervention.