What pathway reduces hemoglobin iron to the physiologic 12 valence state?

The methemoglobin reductase pathway is critical for maintaining hemoglobin in its functional state by reducing ferric iron (Fe3+) back to its ferrous state (Fe2+). When hemoglobin is oxidized, it can form methemoglobin, which is unable to effectively bind and release oxygen. The methemoglobin reductase pathway utilizes NADH as a reducing agent, through the action of the enzyme methemoglobin reductase, to convert methemoglobin back to hemoglobin, thus ensuring that the majority of hemoglobin remains in its functional state to carry oxygen effectively.

In contrast, the other options focus on different processes or pathways within red blood cells. The hexose monophosphate pathway primarily plays a role in providing reducing equivalents and is crucial for maintaining cellular health and the integrity of red blood cells, but it does not specifically reduce hemoglobin iron. The Rapoport-Luebering pathway and the 2,3-BPG shunt are involved in modulating the oxygen affinity of hemoglobin but do not directly address the reduction of iron in hemoglobin itself. Therefore, the methemoglobin reductase pathway is the specific mechanism for reducing hemoglobin iron to its physiologic valence state, making it the correct answer.