What hormone is produced by the adrenal medulla and what is its action?

The hormone produced by the adrenal medulla is norepinephrine, and its primary action involves constricting blood vessels. Norepinephrine is a catecholamine that plays a crucial role in the body's fight-or-flight response. When released into the bloodstream, it causes vasoconstriction, which increases blood pressure and redirects blood flow to essential organs and muscles. This response is vital during stress or emergency situations, enabling the body to respond quickly and effectively.

In contrast, the other choices pertain to different hormones and their effects that do not relate to the adrenal medulla's functions. Thyroxine, produced by the thyroid gland, is primarily responsible for regulating metabolic rate. Cortisol, produced by the adrenal cortex, helps maintain blood glucose levels primarily during stress but does not focus on blood vessel constriction. Insulin, produced by the pancreas, plays a role in increasing protein synthesis, but it is not involved in the actions driven by norepinephrine or produced by the adrenal medulla. Thus, norepinephrine's role in constricting blood vessels accurately connects it to the physiological responses initiated by the adrenal medulla.