Which condition can the determination of blood in CSF confirm?

The presence of blood in cerebrospinal fluid (CSF) is a significant indicator and is most commonly associated with subarachnoid hemorrhage (SAH). When a patient presents with sudden onset of severe headache, often described as a "thunderclap" headache, the presence of blood in CSF can confirm that bleeding has occurred in the subarachnoid space, typically due to a ruptured aneurysm or vascular malformation. This is a critical finding, as SAH requires prompt diagnosis and management to prevent serious complications such as rebleeding or vasospasm.

In contrast, other conditions like cerebral thrombosis, viral encephalitis, and brain tumors do not typically present with blood in the CSF. Cerebral thrombosis primarily involves ischemic processes without early hemorrhage, while viral encephalitis might show pleocytosis with lymphocytic predominance but not blood. Brain tumors can produce atypical cells or metabolic byproducts in the CSF but are not confirmed by the presence of blood. Thus, the detection of blood specifically supports a diagnosis of subarachnoid hemorrhage, making it the correct response in this scenario.