A hospital adds a nitrogen line for surgical tools alongside oxygen and medical air. What single feature most prevents a fatal mix-up of these medical/industrial gases at the outlet?

Explanation

Medical gas station outlets use gas-specific, non-interchangeable indexing so a nitrogen connector cannot fit an oxygen outlet, backed by permanent labeling and color coding. The physical keying, not the label alone, is the primary defense against a lethal wrong-gas connection. A crossover test at commissioning confirms each outlet delivers only its intended gas.