In human skeletal muscle cells, lactic acid fermentation primarily serves to accomplish which of the following during low-oxygen conditions?

• A. Produce oxygen to restore aerobic respiration
• B. Generate the majority of ATP by fully oxidizing glucose to $C{O}_2$ and $H_{2}O$
• C. Convert pyruvate into acetyl-CoA to enter the citric acid cycle
• D. Regenerate $NAD+$ from $NADH$ so glycolysis can continue producing ATP

Answer: (D)

Verified step by step guidance

1. Understand that during low-oxygen (anaerobic) conditions, skeletal muscle cells cannot rely on aerobic respiration, which normally produces ATP by fully oxidizing glucose to carbon dioxide (CO\_2) and water (H\_2O).
2. Recall that glycolysis is the initial pathway for glucose breakdown, producing a small amount of ATP and converting NAD\^+ to NADH in the process.
3. Recognize that for glycolysis to continue, NAD\^+ must be regenerated from NADH; otherwise, glycolysis would halt due to lack of NAD\^+.
4. Know that lactic acid fermentation converts pyruvate (the end product of glycolysis) into lactic acid, and in doing so, oxidizes NADH back to NAD\^+, thus allowing glycolysis to continue producing ATP even when oxygen is scarce.
5. Conclude that the primary role of lactic acid fermentation in skeletal muscle cells under low-oxygen conditions is to regenerate NAD\^+ from NADH, enabling continued ATP production via glycolysis.