Which of the following agents is categorized as a positive ionotropic agent?

A positive inotropic agent is a substance that increases the strength and force of the heart's contractions. Dopamine fits this definition because it can enhance cardiac output by stimulating beta-adrenergic receptors, particularly in higher doses. This stimulation leads to increased myocardial contractility, which is particularly beneficial in acute heart failure situations where improved cardiac function is necessary.

In contrast, beta blockers, which are designed to decrease heart rate and myocardial contractility, and antiarrhythmics, which are primarily used to manage abnormal heart rhythms, do not serve as positive inotropic agents. Acetylcholine, a neurotransmitter, generally has a negative effect on heart rate and contractility when acting on the heart's parasympathetic system. Hence, dopamine stands out as the agent that increases heart muscle contraction strength, making it the correct answer in identifying a positive inotropic agent.