The teen was riding an Q25 bus at 82nd Drive and Parsons Boulevard around 3:50 p.m. when the unhinged rider suddenly knifed him once in the stomach, cops said.

The senior, identified Wednesday as Lydia Marzan, was discovered lifeless around 6:50 a.m. Tuesday, sprawled out on the sidewalk in front of the national historic landmark on Southern Boulevard in Bedford Park, police said.