**Core Concept**
Pralidoxime is a medication used to treat organophosphate and certain carbamate poisoning by reactivating acetylcholinesterase, the enzyme responsible for breaking down acetylcholine in the nervous system. Its efficacy is contingent upon the specific type of poisoning.

**Why the Correct Answer is Right**
Carbaryl is a carbamate insecticide, and pralidoxime is ineffective against carbamate poisoning because carbamates form a stable carbamoylated complex with acetylcholinesterase, which cannot be easily reversed by pralidoxime. In contrast, organophosphorus compounds form a phosphorylated complex with acetylcholinesterase, which can be effectively reactivated by pralidoxime.

**Why Each Wrong Option is Incorrect**
**Option A:** Pralidoxime is indeed effective against organophosphorus poisoning by reactivating acetylcholinesterase, thus making this option incorrect.
**Option C:** Given that pralidoxime is ineffective against carbamate poisoning, this option is also incorrect.
**Option D:** Since carbaryl is a specific example of carbamate poisoning where pralidoxime is ineffective, this option is incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pralidoxime is ineffective against carbamate poisoning, and treatment for carbamate poisoning primarily involves atropine and supportive care.

**✓ Correct Answer: B. Carbaryl**