 Later , in the lab at Harvard Walter 's autopsy reveals a synthetic compound in Mark 's blood , though any link to the cuts is not yet known to them . Olivia receives an email from someone who claims to be John Scott , listing an address for her to visit . Upon arriving she finds boxes , one of which contains a group of toads . At the lab , Walter finds that the toads contain a " psychoactive compound " , a <unk> that affects the fear center of the brain . They conclude that Mark ’ s brain was so convinced of something happening to his body that actual physical marks appeared . Mark was infected with a large <unk> , leading them to attribute his death to murder . 
