Histotoxic hypoxia occurs when

• A. Toxins interfere with tissue oxygen utilization
• B. Oxygen content in blood is reduced
• C. There is a failure to deliver oxygen to tissues due to low atmospheric pressure
• D. There is an excess amount of oxygen at tissues

Answer: (A)

Histotoxic hypoxia happens when toxins prevent cells from using oxygen. Even though oxygen is arriving to tissues normally, cellular respiration is blocked, so the oxygen can’t be utilized to make energy. Toxins like cyanide disrupt mitochondrial enzymes (such as cytochrome c oxidase) in the electron transport chain, halting oxidative phosphorylation. So the problem is at the tissue level's ability to use oxygen, not at delivery or availability of oxygen to the blood.

That’s why this fits best: the issue is not that there’s less oxygen in the blood or that oxygen isn’t reaching the tissues, but that the tissues can’t metabolize the oxygen that is there. In contrast, reduced oxygen content in blood or low atmospheric pressure affects delivery or availability of oxygen (hypoxic conditions), and having too much oxygen at the tissues isn’t what defines histotoxic hypoxia.