Post-traumatic vocal disarticulation
Post-traumatic vocal disarticulation is a condition involving the vocal chords of a human being. After experiencing extreme stress or trauma, a person diagnosed with this condition is rendered completely unable to speak, even if there is no catastrophic injury to their actual vocal chords. The condition is purely psychological, although after extended periods of silence an individual suffering from post-traumatic vocal disarticulation may experience significant difficulty speaking coherently even if they overcome the condition on a psychological level.
SPARTAN-III supersoldier Lucy-B091 was diagnosed with this condition at age 12 after her company was killed in Operation: TORPEDO, leaving her and Tom-B292 as the only survivors out of 300. This shook her terribly and she would remain silent for years afterward, though she managed to speak a few words after being exceedingly agitated at Dr. Catherine Halsey for harassing the Huragok Prone to Drift.