What prevents high-pressure oxygen from entering the downstream system if the pressure reducer fails?

The correct choice is the pressure relief valve. In the context of cabin atmosphere control systems, a pressure relief valve is designed to prevent excessive pressure from entering downstream equipment when the pressure reducer fails.

When there is a fault in the pressure reduction mechanism, the pressure relief valve acts as a safeguard by venting excess pressure. This control helps to maintain a safe pressure level within the system, ensuring that the downstream components are not exposed to pressures beyond their rated limits.

While other components such as emergency shutoff valves, check valves, and pressure gauges play important roles in the overall system, they do not directly provide the specific function of relieving excess pressure in the event of a pressure reducer failure. The emergency shutoff valve is more focused on stopping the flow altogether, the check valve prevents backflow rather than managing excess pressure, and the pressure gauge simply measures pressure without mitigating it. Thus, the pressure relief valve is the critical component that protects the downstream system from high-pressure oxygen in case of a failure in the pressure reduction system.