Anxiety

 is Disc 2, Track 4 on the Xenosaga Original Soundtrack and Disc 2, Track 20 on the album. ('Anxiety' is an unofficial translation, as seen in the VGMdb listing of the album.)

It plays in the following cutscenes:
 * Virgil shows up in the Realian Maintenance Centre on the Woglinde.
 * Virgil prepares to remotely control the Realians to hold off the Gnosis during their attack on the Woglinde.
 * Margulis talks with MOMO in her cell on Pleroma.
 * A hooded figure hands Cherenkov KOS-MOS' pod control device (flashback).
 * Albedo shows up and taunts Margulis on Pleroma.
 * Jr. finds the remains of the Zohar Emulator's security ring in the ruins of the Woglinde.
 * Cherenkov has flashbacks of the Gnosis attacking him onboard the Elsa, and injects himself with an unknown device.
 * Cherenkov notices his hand becoming transparent, and fails to administer his medication.
 * Shion wakes up inside the Cathedral Ship.
 * Cherenkov hallucinates scenes from his past life inside the Cathedral Ship.
 * The party notices that the Cathedral Ship's interior seems to have human-made signs.
 * Cherenkov sees visions of people he was responsible for killing while standing in front of the Zohar Emulator in the Cathedral Ship.
 * The party enters the room with partially-Gnosified people on the Durandal.
 * Shion and Jr. confront their past painful memories inside the Old Church during the dive into KOS-MOS' subconscious domain.
 * Margulis gives the order for Albedo to use the Song of Nephilim.
 * MOMO sees the remains of the Kirschwassers before Albedo confronts her in the Song of Nephilim.
 * The Blue Testament confronts the party in the Song of Nephilim.