**Core Concept**
The question is testing the student's knowledge of local anesthetic adjuvants, specifically an enzyme that can be added to local anesthetics (LAs) to enhance their action. This concept is crucial in the management of pain and anesthesia, where the goal is to achieve rapid onset and wide spread of the anesthetic effect.

**Why the Correct Answer is Right**
The enzyme that can be added to LAs to speed both onset and area of anesthesia is **esterase**, specifically **esterase inhibitors** such as esterases that break down ester-type local anesthetics into their active metabolites. By inhibiting these esterases, the local anesthetic remains active for longer, leading to a faster onset and wider spread of anesthesia. This is particularly useful in procedures where rapid anesthesia is required, such as in emergency situations or during minor surgical procedures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the question. The correct answer is an enzyme, not a type of local anesthetic or anesthetic agent.
* **Option B:** This option is incorrect because cholinesterase inhibitors are used to prevent nerve damage caused by local anesthetics, not to speed up their onset and area of action.
* **Option C:** This option is incorrect because sodium channel blockers are the mechanism of action of local anesthetics, not an enzyme that can be added to them.

**Clinical Pearl / High-Yield Fact**
When using local anesthetics with esterase inhibitors, it's essential to be aware of the potential for toxicity, particularly to the central nervous system (CNS) and cardiovascular system (CVS). Monitor patients closely for signs of toxicity, such as CNS excitation or CVS depression.

**Correct Answer: C. Esterase. Esterase inhibitors can be added to LAs to speed both onset and area of anesthesia.**