The primary cause of histotoxic hypoxia in a crew member is which of the following?

Histotoxic hypoxia occurs when the body's tissues are unable to utilize oxygen effectively, even when adequate oxygen is available in the blood. The primary cause of histotoxic hypoxia is the presence of certain toxic substances that interfere with cellular respiration.

Cyanide is known to inhibit the enzyme cytochrome c oxidase in the mitochondrial electron transport chain, which is essential for aerobic metabolism. This inhibition prevents cells from using oxygen effectively, despite normal oxygen levels being present in the bloodstream. Therefore, cyanide directly affects the body's ability to utilize oxygen at the cellular level, leading to histotoxic hypoxia.

Other substances like alcohol may cause various forms of hypoxia or alter the body's response to hypoxia, but they do not specifically block the use of oxygen in the way cyanide does. Carbon dioxide is a byproduct of respiration and does not cause histotoxic hypoxia, while ozone, although harmful to the respiratory system, does not directly impact the tissues’ ability to utilize oxygen. Therefore, cyanide is definitively recognized as the primary agent leading to histotoxic hypoxia.