An individual in poor condition runs a marathon after being dared to do so by a friend. Halfway through the marathon, he collapses, and is taken to an emergency department. Stat arterial blood gases demonstrate PO2 100 mmHg, PCO2 42 mmHg, and pH 7.30. Follow-up serum chemistries demonstrate a lactate level of 7 mmol/liter (reference range 0.5 to 2.2 mmol/liter). This lactate is the product of which of the following biochemical pathways?

A. Anaerobic glycolysis
B. Beta oxidation of fatty acids
C. Citric acid cycle
D. Mitochondria oxidative phosphorylation
E. Pentose phosphate shunt

A. Anaerobic glycolysis ?

Good!

The correct answer is A. In severe exercise, particularly in individuals in poor physical condition, the oxygen demands of the skeletal muscle may exceed the ability of the heart and lungs to provide oxygen. In this setting, the muscles switch to anaerobic glycolysis, and the pyruvate that is produced (and cannot be used as acetyl-CoA in the citric acid cycle) is converted to lactate. Much of the lactic acid thereby produced is released into the blood stream, where it causes a metabolic acidosis. In addition to severe exercise, lactic acidosis can also be seen in the settings of tissue hypoxia (seizures, cardiac failure, hypotension, carbon monoxide poisoning, severe anemia), drug toxicity and toxins (phenformin, catecholamines, salicylate, isoniazide, cyanide), congenital defects in gluconeogenic enzymes, and many severe illnesses.
Beta oxidation of fatty acids (choice B) can produce acetyl CoA, which may be converted to ketone bodies such as acetone and â-hydroxybutyrate if the acetyl CoA is not used in the citric acid cycle.

The citric acid cycle (choice C) produces CO2, NADH, and FADH2.

Mitochondrial oxidative phosphorylation (choice D) produces water and ATP.

The pentose phosphate shunt (choice E) is an important producer of NADPH and ribose-5-phosphate.