What is the primary cause of fainting (syncope)?

Fainting, or syncope, primarily occurs due to a temporary reduction of blood flow to the brain. When the brain does not receive enough blood, which carries oxygen and nutrients, it can result in a loss of consciousness. Several factors can lead to this reduced blood flow, such as standing up too quickly, dehydration, emotional stress, or medical conditions that affect circulation.

In the context of the other choices, while a temporary reduction of blood flow can occur in various parts of the body and might lead to fainting under certain circumstances, the critical factor specifically related to syncope is the lack of adequate blood flow to the brain. Though overexertion can lead to conditions where fainting might occur, it is not the primary cause. Instead, it's the direct impact on brain perfusion that typically results in a fainting episode.