Acidic pH of muscles is due to?

Hint: Cellular respiration produces ATP from pyruvate in the mitochondria. Glycolysis transforms glucose into pyruvate, water, and NADH, and produces two molecules of ATP. Phosphocreatine delivers phosphates to the ADP molecules and produces high-energy ATP molecules.

Complete answer:
Lactic acid is created and accumulated in the muscle fibre under high energy demand, rapid variations of energy requirement and deficient supply of O2. Due to intense exercise and activity, muscles sustain fatigue, and muscle pH decreases around 6.4-6.6 due to the accumulation of lactic acid. In aerobic respiration, pyruvate is produced by the process of glycolysis. It is then converted into additional ATP molecules in the mitochondria by Krebs cycle. Due to insufficient oxygen, pyruvate cannot enter the Krebs cycle and instead gets accumulated in the muscle fibre. Pyruvate is continuously processed into lactic acid. Lactic acid production is increased with pyruvate accumulation. The lactic acid accumulation in muscle tissue reduces pH and hence makes it more acidic and produces the stinging feeling in muscles while exercising. This inhibits anaerobic respiration and induces fatigue.

So, the correct answer is option A.

Additional Information:
Lactic acidosis is a medical condition which involves the build-up of lactate in the body. It results from an underlying acute or chronic medical condition, medication, or poisoning. The symptoms may include nausea, vomiting, labored and deep breathing, and weakness.

Note: ATP is essential for muscle contraction. The four sources of ATP are available in muscle fibres: free ATP molecules, phosphocreatine, glycolysis, and cellular respiration. Some small amount of free ATP is present in the muscle for immediate usage.