Which of the following describes a warning sign of a stroke?

A severe headache with no known cause is indeed a significant warning sign of a stroke. This type of headache, often described as the worst headache of one's life, can indicate a hemorrhagic stroke, where there is bleeding in or around the brain. It is crucial to recognize this symptom as it may signal an urgent medical situation requiring immediate attention.

The other choices do indicate health issues, but they are not primary warning signs of a stroke. For example, a rapid heartbeat with chest pain can indicate a heart-related problem rather than a stroke. Sudden onset of nausea and vomiting can also be symptoms of various conditions, including gastrointestinal issues or migraines, but they are not specific indicators of a stroke. Chronic fatigue and weakness can result from numerous long-term health conditions or lifestyle factors, but they do not typically signify an acute stroke event. Recognizing the correct signs is essential for timely intervention and treatment in stroke cases.