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Tension Before a Confrontation (対決前緊張) is an ominous theme in Metroid Fusion. As implied by the title, it is heard when about to confront a boss character in the vicinity, often foreshadowing the presence of a boss character, as well as the immediate aftermath of a boss battle, and is heard for all boss rooms save for those of the scientist (due to the character being fought during a countdown), Nettori, the second Security Robot B.O.X. boss fight, the SA-X boss fight, and the Omega Metroid boss fight.
The theme can be heard here.