**Core Concept:** Synergistic block is the interaction between two or more drugs or anesthetic agents that leads to a greater than expected effect when combined. In the context of neuromuscular blockers, antagonists like neostigmine and edrophonium can reverse partial neuromuscular block caused by other drugs.

**Why the Correct Answer is Right:** When given together, a neuromuscular blocking agent like succinylcholine and a non-depolarizing neuromuscular blocking agent like rocuronium, the resulting block is greater than the sum of their individual effects. This is an example of synergism. A synergistic block leads to a more profound muscle relaxation and faster onset of action.

**Why Each Wrong Option is Incorrect:**

A. **Neither synergism nor antagonism:** This is incorrect because the question specifically mentions synergism in the options.
B. **Antagonism:** Antagonism refers to drugs that oppose or counteract each other, which contradicts the question's description of synergism.
C. **Neither antagonism nor synergism:** This is also incorrect because the question presents synergism as one of the options.
D. **Neither antagonism nor synergism:** Similar to options A and C, this choice is incorrect as synergism is mentioned in the options.

**Clinical Pearl / High-Yield Fact:** Understanding synergistic interactions between anesthetic agents is crucial for selecting the appropriate combination to achieve optimal patient care. For example, combining succinylcholine with a non-depolarizing agent like rocuronium results in a quicker onset of action and deeper muscle relaxation, making it an effective choice for rapid-sequence induction in emergency situations.