What best explains the symptoms experienced by the 25-year-old woman during her sprint training?

The symptoms experienced by the woman during her sprint training can be best explained by the regeneration of NAD+ from NADH, which leads to the production of lactic acid. During intense exercise such as sprinting, cells undergo anaerobic metabolism due to insufficient oxygen delivery to meet the energy demands. In this anaerobic state, pyruvate, which is a product of glycolysis, is converted into lactic acid to regenerate NAD+. This process is essential because NAD+ is required for glycolysis to continue producing ATP.

As sprinting is a high-intensity activity, the rapid conversion of pyruvate to lactic acid (and the resulting increase in lactic acid levels) is what typically leads to the fatigue and muscle burning sensations that athletes often experience. Therefore, the accumulation of lactic acid is a key factor in the symptoms the woman is experiencing during her sprint training.

The other choices do not adequately explain the physiological changes during such intense exercise. For instance, fluctuations in sodium-proton antiporter activity and oxygen delivery are not primary mechanisms responsible for the intense symptoms during sprinting compared to the direct contribution of lactic acid production.