Fomepizole is a selective antidote for poisoning with:
**Core Concept**
Fomepizole is a selective antidote for methanol or ethylene glycol poisoning. It acts by inhibiting the enzyme alcohol dehydrogenase, which is responsible for the conversion of methanol to its toxic metabolites. By blocking this enzyme, fomepizole prevents the formation of formaldehyde and formic acid, which are the primary toxic compounds responsible for the clinical manifestations of methanol poisoning.
**Why the Correct Answer is Right**
Fomepizole selectively inhibits alcohol dehydrogenase, specifically the isoenzyme that metabolizes methanol. This inhibition prevents the formation of toxic metabolites, such as formic acid, which can cause visual disturbances, metabolic acidosis, and even death. By administering fomepizole promptly, clinicians can reduce the severity of methanol poisoning and prevent long-term sequelae.
**Why Each Wrong Option is Incorrect**
**Option A:** MAO inhibitors are a class of antidepressants that work by inhibiting monoamine oxidase enzymes, which are involved in the breakdown of neurotransmitters such as serotonin and dopamine. Fomepizole is not indicated for MAO inhibitor overdose.
**Option B:** Ethyl alcohol, or ethanol, is metabolized by alcohol dehydrogenase, but fomepizole is not typically used as an antidote for ethanol poisoning. Ethanol is often used as a treatment for methanol poisoning, as it can competitively inhibit alcohol dehydrogenase and reduce the formation of toxic metabolites.
**Option D:** Tricyclic antidepressants (TCAs) are a class of medications that work by inhibiting the reuptake of serotonin and norepinephrine. Fomepizole is not indicated for TCA overdose, as its mechanism of action does not involve the inhibition of TCAs.
**Clinical Pearl / High-Yield Fact**
When treating methanol poisoning, it's essential to remember that fomepizole is most effective when administered promptly, within the first few hours after ingestion. Delayed treatment can lead to increased toxicity and worse outcomes.
**Correct Answer:**
✓ Correct Answer: C. Methyl alcohol