The child with developmental delays and characteristic neurological findings has a familial history suggestive of a metabolic disorder. Which metabolic pathway is likely affected if complex II activity is diminished?

• A. Beta-oxidation of fatty acids
• B. Glycolysis
• C. Electron transport chain function
• D. Urea cycle

Answer: (C)

In this scenario, the diminished activity of complex II points directly to the electron transport chain, which is the pathway responsible for aerobic respiration in mitochondria. Complex II, also known as succinate dehydrogenase, plays a crucial role in both the citric acid cycle and the electron transport chain. It functions to transfer electrons from succinate to ubiquinone, which is an essential step in the process of generating adenosine triphosphate (ATP) through oxidative phosphorylation.

When complex II activity is decreased, the electron transport chain's ability to function properly is impaired, leading to a reduction in ATP production and potentially causing an accumulation of metabolic byproducts. This dysfunction can contribute to a variety of neurological symptoms and developmental delays, particularly in a child, as the brain has high energy demands.

In terms of the other pathways listed, while they are critical for overall metabolism, they do not directly involve complex II. For instance, beta-oxidation of fatty acids focuses on breaking down fatty acids for energy, glycolysis is the process of glucose breakdown into pyruvate for initial ATP release, and the urea cycle is responsible for nitrogen waste disposal. None of these pathways are directly linked to complex II, which makes the identification of the