Anaerobic sprint metabolism — During vigorous exercise when oxygen delivery is limiting, pyruvate produced by glycolysis is converted into what product?

Introduction:
High-intensity exercise can outpace oxygen delivery, forcing muscle cells to regenerate NAD+ without the electron transport chain. This question asks which metabolite pyruvate becomes under such conditions to keep glycolysis running.

Given Data / Assumptions:

• Glycolysis requires NAD+ for the glyceraldehyde-3-phosphate dehydrogenase step.
• Oxygen is limited, restricting oxidative phosphorylation and NADH oxidation.
• Cells must reoxidize NADH to NAD+ anaerobically.

Concept / Approach:
Lactate dehydrogenase reduces pyruvate to lactate while oxidizing NADH to NAD+. This rapid fermentation step sustains ATP production by glycolysis when mitochondrial respiration is constrained. Lactate can later be exported to blood, taken up by the liver for gluconeogenesis (Cori cycle), or oxidized when oxygen becomes available.

Step-by-Step Solution:

Verification / Alternative check:
Measurable increases in blood lactate and hydrogen ions correlate with intense exercise; post-exercise recovery clears lactate as oxidative metabolism resumes or via gluconeogenesis in the liver.

Why Other Options Are Wrong:

• Acetate/Acetyl-CoA: require PDH and oxygen-supported respiration to proceed usefully; not the primary anaerobic fate.
• Monosodium phosphate: a buffer component, not a metabolic product of pyruvate.
• Pyruvic acid unchanged: would not regenerate NAD+; glycolysis would halt.

Common Pitfalls:
Viewing lactate as a “waste” only. It is an important metabolic intermediate and fuel shuttle among tissues (lactate shuttle concept).

Final Answer:
Lactate.