A patient suffering from severe chest pain is admitted to the ER. After a series of tests, it is found that blood is flowing back into the left atrium as it is pumped into the left ventricle. Which valve is responsible for preventing this from happening?

The mitral valve sits between the left atrium and the left ventricle. When the left ventricle contracts to pump blood onward, the mitral valve closes to prevent blood from flowing backward into the left atrium. Therefore, the correct choice is the mitral valve.

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