**Core Concept**
Isoniazid poisoning is a medical emergency that requires prompt treatment. Isoniazid is a hydrazide derivative used in the treatment of tuberculosis. It can cause severe hepatotoxicity and seizures, especially in overdose situations.

**Why the Correct Answer is Right**
The treatment of isoniazid poisoning involves the use of pyridoxine (vitamin B6) and other supportive measures. Pyridoxine is a cofactor for the enzyme glutamic acid decarboxylase, which is involved in the synthesis of gamma-aminobutyric acid (GABA). Isoniazid inhibits this enzyme, leading to an increase in extracellular glutamate and a decrease in GABA levels, resulting in seizures. Administering pyridoxine helps to replenish the cofactor, thereby reducing the risk of seizures.

**Why Each Wrong Option is Incorrect**
**Option A:** Flumazenil is a benzodiazepine antagonist used to reverse benzodiazepine overdose. It is not indicated in the treatment of isoniazid poisoning.
**Option B:** Activated charcoal may be used to absorb isoniazid in cases of overdose, but it is not a specific antidote for isoniazid poisoning.
**Option C:** N-acetylcysteine is used to treat acetaminophen (paracetamol) overdose, not isoniazid poisoning.

**Clinical Pearl / High-Yield Fact**
Remember that isoniazid poisoning can cause seizures, and administering pyridoxine can help prevent these seizures. Also, activated charcoal may be used to absorb isoniazid, but it is not a specific antidote.

**Correct Answer: D. Pyridoxine (Vitamin B6)**