A patient presents with large anion gap acidosis and flank pain after ingesting a substance. Which substance was most likely ingested?

The scenario describes a patient with large anion gap acidosis and flank pain, which suggests a metabolic disturbance often associated with the ingestion of specific toxic substances. Ethylene glycol is known to cause large anion gap metabolic acidosis due to its metabolism into glycolic acid and oxalic acid, both of which contribute to acidemia. Additionally, the flank pain indicates the potential formation of calcium oxalate crystals, a common complication associated with ethylene glycol toxicity, which can lead to renal injury.

Methanol also results in large anion gap acidosis and can cause flank pain, but it typically has more pronounced ocular symptoms and its metabolic profile differs slightly. Acetaminophen overdose primarily causes hepatotoxicity rather than significant acidosis, and iron toxicity primarily leads to gastrointestinal symptoms and not a large anion gap. Each of these substances has distinct clinical presentations, but ethylene glycol's characteristic effects align closely with the symptoms described in the question.