Story related; cannot be missed.
*** Spoiler - click to reveal ***After leaving the community and reuniting with Clementine, she have a flashback to her days with the New Frontier. During this flashback, Clementine will attempt to save AJ to no avail (whether you choose to use the medicine or not) and be kicked out of the group. After she is escorted away, the flashback will end and the achievement will unlock.