Which of the following would likely be a result of administering a blocker drug?

Administering a blocker drug typically interferes with the action of specific neurotransmitters in the body. In the case of a blocker drug that affects the cardiovascular system, it often targets receptors that regulate heart rate and contractility.

When a blocker drug is given, such as a beta-blocker, it inhibits the effects of adrenaline (epinephrine) and norepinephrine on heart receptors, resulting in a reduction of heart rate. This is particularly beneficial in treating conditions like hypertension and certain heart disorders, where lowering the heart rate can reduce the workload on the heart and improve overall cardiac efficiency.

The other options reflect physiological changes that would not be associated with the action of a blocker drug. For example, increased muscle contraction and enhanced neurotransmission generally represent stimulatory effects, while improved synaptic dilation would imply enhanced blood flow, which contrasts with the inhibitive nature of blocker drugs. Thus, reduction in heart rate is the expected and correct outcome of administering a blocker drug.