Which of the following decreases minimum alveolar concentration (MAC)?

Decreasing the minimum alveolar concentration (MAC) is an important pharmacological concept, particularly regarding anesthetic agents. The correct answer in this case is pregnancy.

During pregnancy, hormonal changes and physiological adaptations occur, including increased blood volume, altered body fluid distribution, and variations in metabolism. These factors can lead to an increased sensitivity to anesthetic agents, which subsequently lowers the MAC. Consequently, pregnant individuals require lower concentrations of anesthetic to achieve the same level of anesthesia, reflecting the decreased MAC during this condition.

The other options do not lead to a decrease in MAC. For instance, hyperthermia generally increases metabolic rate and can raise the MAC threshold. Cocaine intoxication does not typically alter the MAC substantially and may even increase anesthetic requirements due to its stimulant properties. Chronic ethanol abuse can result in increased tolerance to anesthetic medications, potentially leading to a higher MAC requirement rather than a decrease. Therefore, the changes in physiological status associated with pregnancy are what primarily contribute to the lowered MAC.