What condition is characterized by a sudden, severe headache with no known cause?

The condition characterized by a sudden, severe headache with no known cause is subarachnoid hemorrhage. This type of headache is often described as a "thunderclap" headache, indicating its abrupt onset and intensity, which distinguishes it from other types of headaches. Subarachnoid hemorrhage occurs when there is bleeding in the space surrounding the brain, usually due to the rupture of a cerebral aneurysm or a traumatic injury. The sudden nature of the headache, along with accompanying symptoms such as nausea, vomiting, and possibly loss of consciousness, points to this serious and emergent medical condition.

In contrast, migraines typically present with recurrent, moderate to severe headaches that may be accompanied by other symptoms such as nausea, light sensitivity, and aura. Tension headaches usually develop gradually and are characterized by a more tightening or pressure-like sensation. Cluster headaches occur in cyclical patterns and are marked by intense pain around one eye, as well as other symptoms such as nasal congestion. While these headaches can be severe, their presentation and clinical features differ significantly from those associated with subarachnoid hemorrhage, making the latter the correct answer in this scenario.