What metabolic by-product accumulates when glucose breakdown occurs in the absence of oxygen?

When glucose breakdown occurs without the presence of oxygen, the process shifts from aerobic respiration to anaerobic fermentation. During this anaerobic process, glucose is converted into pyruvate through glycolysis. However, in the absence of oxygen, the pyruvate is not sent to the mitochondria for further breakdown through the Krebs cycle. Instead, it is converted into lactic acid. This accumulation of lactic acid is what primarily characterizes anaerobic metabolism in human muscles, especially during high-intensity exercise when oxygen supply is limited.

Lactic acid can lead to a temporary decrease in pH within the muscle cells, contributing to the sensation of fatigue. The body eventually attempts to clear this lactic acid by converting it back to glucose in the liver through a process called gluconeogenesis once oxygen becomes available again. The presence of lactic acid is a key indicator of anaerobic respiration, differentiating it from more typical aerobic processes, which result primarily in carbon dioxide and water as by-products.

Thus, the correct answer is lactic acid, as it directly correlates with glucose breakdown occurring without oxygen.