Match each patient's clinical presentation and laboratory findings to the most appropriate… — USMLE Step 3

Match each patient's clinical presentation and laboratory findings to the most appropriate toxicological diagnosis on the differential.

An obtunded 54-year-old man is brought to the emergency department after being found unresponsive in a garage. Laboratory evaluation demonstrates an arterial $pH$ of $7.15$ , a serum bicarbonate of $10$ mEq/L, an anion gap of $28$ mEq/L, and a significant serum osmolar gap. Funduscopic examination reveals hyperemic optic discs.Methanol ingestion
A 36-year-old woman is evaluated in the intensive care unit for progressive altered mental status and acute oliguria. Her laboratory results show an arterial $pH$ of $7.20$ , an anion gap of $26$ mEq/L, and an elevated osmolar gap. Microscopy of her urine sediment demonstrates numerous rectangular, envelope-shaped crystals.Ethylene glycol ingestion
A 24-year-old graduate student is admitted to the medical ward with acute confusion, tachypnea, and vomiting. He complains of a persistent, loud ringing in his ears. An arterial blood gas analysis reveals a $pH$ of $7.42$ , $PaCO_2$ of $20$ mm Hg, and a calculated anion gap of $22$ mEq/L.Salicylate toxicity
A 41-year-old man with a history of alcohol use disorder is seen at a community health clinic presenting with severe lethargy and ataxia. His vital signs are within normal limits. A chemistry panel shows a normal anion gap but a markedly elevated osmolar gap. Urinalysis is strongly positive for ketones, although his blood glucose is $92$ mg/dL.Isopropyl alcohol ingestion

Answer

The 54-year-old with hyperemic optic discs matches Methanol ingestion; the 36-year-old with oliguria and envelope-shaped crystals matches Ethylene glycol ingestion; the 24-year-old with tinnitus and a mixed acid-base disorder matches Salicylate toxicity; and the 41-year-old with ketonuria but a normal anion gap matches Isopropyl alcohol ingestion.

Methanol produces formic acid causing profound retinal toxicity. Ethylene glycol produces glycolic and oxalic acids, causing acute kidney injury and calcium oxalate crystalluria. Salicylates uncouple oxidative phosphorylation and stimulate the medullary respiratory center, causing a mixed acid-base disorder with tinnitus. Isopropyl alcohol is metabolized to acetone, causing an osmolar gap and ketonuria without elevating the anion gap.

Step-by-Step Solution

Evaluate the clinical vignette describing the 54-year-old man.

Identify a high anion gap metabolic acidosis, an osmolar gap, and specific end-organ damage (hyperemic optic discs).

These findings accurately distinguish methanol toxicity, which uniquely targets the retina and optic nerve, from other toxic alcohols.

Evaluate the clinical vignette describing the 36-year-old woman.

Identify a high anion gap metabolic acidosis, an osmolar gap, and acute oliguria with envelope-shaped crystals.

These findings pinpoint ethylene glycol toxicity due to the formation of calcium oxalate crystals leading to acute kidney injury.

Evaluate the clinical vignette describing the 24-year-old student.

Identify a mixed acid-base disorder (primary respiratory alkalosis and high anion gap metabolic acidosis) accompanied by tinnitus.

This is the classic, pathognomonic presentation for salicylate (aspirin) toxicity.

Evaluate the clinical vignette describing the 41-year-old man.

Identify central nervous system depression with an elevated osmolar gap and prominent ketonuria, but notably a normal anion gap.

This isolates isopropyl alcohol ingestion, as it is the only common toxic alcohol that produces ketosis without a concurrent anion gap metabolic acidosis.

Key Concept

Differentiating toxic alcohol and salicylate ingestions requires analyzing acid-base status, osmolar gaps, and specific end-organ manifestations such as retinal toxicity, crystalluria, or ototoxicity.

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