Which of the following describes a parasympathomimetic agent?

A parasympathomimetic agent is one that mimics the effects of the parasympathetic nervous system. The parasympathetic system is involved in 'rest and digest' functions, and its activation typically leads to responses such as increased secretion, decreased heart rate, and bronchoconstriction. In the context of respiratory therapy, parasympathomimetic agents can cause bronchoconstriction and increase mucus secretion in the airways, which is important in understanding their effects on respiratory conditions.

This choice correctly describes the action of parasympathomimetic agents, providing a clear distinction from the sympathetic system, which is associated with 'fight or flight' responses and notably leads to bronchodilation. In contrast, the other options describe actions typical of different drug classes or systems, such as sympathomimetics for option A, which mimic sympathetic effects, or agents that inhibit smooth muscle contraction in option D, which do not align with the role of parasympathomimetics.