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.

we need an attachment to see what you are talking about.