Available when the relationship with Lexi has advanced enough, after Lexi asks Ryder if she is uptight. Next time you land on the Nexus, Suvi will call Ryder about the whereabouts of the doctor. This starts the mission.
Find Lexi on the NexusEdit
The mission title gives a hint on where to find her: the Vortex. Speak with her about her worries to complete the mission.
None (except advancing the relationship with her).
Suvi does not always call Ryder, leaving it up to chance that the player will find Lexi in the Vortex, which will still complete the mission as it is added to the Journal regardless. (Confirmed on PS4.)