**Core Concept**
Ethylene glycol, a common ingredient in antifreeze, is metabolized into toxic compounds that cause metabolic acidosis, renal failure, and central nervous system depression.

**Why the Correct Answer is Right**
Ethylene glycol is first converted to glycoaldehyde by alcohol dehydrogenase, which is then further metabolized to oxalic acid, glycolic acid, and formic acid. These metabolites cause cellular damage, leading to metabolic acidosis, renal failure, and central nervous system depression. The renal failure is due to the deposition of calcium oxalate crystals in the renal tubules.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the primary cause of toxicity is not an allergic reaction, but rather the metabolic byproducts of ethylene glycol.

**Option B:** This option is incorrect because the primary cause of toxicity is not an overdose of ethylene glycol, but rather the metabolic byproducts that are produced when it is ingested.

**Option C:** This option is incorrect because the primary cause of toxicity is not an anaphylactic reaction, but rather the metabolic byproducts of ethylene glycol.

**Clinical Pearl / High-Yield Fact**
The treatment of ethylene glycol poisoning involves the administration of fomepizole or ethanol to inhibit the conversion of ethylene glycol to its toxic metabolites, as well as supportive care to manage metabolic acidosis and renal failure.

**Correct Answer: A. Glycoaldehyde is the first metabolite of ethylene glycol, leading to toxic effects.