While waiting for the subway train to arrive, mark suddenly felt a wave of anxiety come over him. he felt dizzy, broke out into a cold sweat, and thought he was going crazy. the other people around him did not seem real. most likely, mark experienced a:

Panic attack

Explanation: Panic attacks are considered to be an anxiety disorder where a person experiments feelings of anguish, anxiety, despersonalization (not feeling himself), etc. Panic attacks are always triggered by a situation of thoughts that leads to an intense panic and fear of dying, this causes trouble breathing that gives more evidence of a "possible death" causing even more symptoms; it's a cycle that can be broken by taking the person to an open enviroment and having personal contact with other people.