A 58-year-old man with diabetes mellitus presents to the emergency department 6 hours after accidental ingestion of ethylene glycol. He is confused, has a respiratory rate of 28/min, and blood pH is 7.18. Serum creatinine is 2.8 mg/dL (baseline 0.9 mg/dL). Urinalysis shows muddy brown casts and granular casts. What is the most appropriate immediate next step in management?

Explanation

## Clinical Context This patient presents with acute tubular necrosis (ATN) secondary to ethylene glycol toxicity, manifested by acute kidney injury (AKI), metabolic acidosis, and characteristic muddy brown casts on urinalysis. ## Pathophysiology of ATN in Toxin Exposure **Key Point:** Ethylene glycol is metabolized to oxalic acid and glycolic acid, which cause direct tubular epithelial injury and crystal-induced obstruction. The muddy brown casts (mixture of cellular debris, haemoglobin, and myoglobin) are pathognomonic for ATN. ## Management Algorithm for ATN with Metabolic Acidosis ```mermaid flowchart TD A[ATN from ethylene glycol toxicity]:::outcome --> B{Metabolic acidosis present?}:::decision B -->|Yes| C[Initiate fomepizole + IV sodium bicarbonate]:::action B -->|No| D[Supportive care + fluid resuscitation] C --> E[Arrange urgent haemodialysis]:::action E --> F[Remove toxin + correct acidosis + manage AKI]:::outcome D --> G[Monitor renal function] ``` ## Rationale for Correct Answer **High-Yield:** The immediate priority in ethylene glycol poisoning with ATN is: 1. **Block further metabolism** — fomepizole inhibits alcohol dehydrogenase, preventing conversion to toxic metabolites 2. **Correct acidosis** — sodium bicarbonate alkalinizes urine, reducing crystal precipitation and enhancing renal clearance 3. **Remove toxin** — haemodialysis is the definitive treatment, removing both ethylene glycol and its toxic metabolites **Clinical Pearl:** Fomepizole + bicarbonate should be started immediately while arranging dialysis; do not delay these interventions waiting for dialysis availability. ## Why Each Step Matters - **Fomepizole:** Competitive inhibitor of alcohol dehydrogenase; prevents formation of oxalic acid (the most nephrotoxic metabolite) - **Sodium bicarbonate:** Alkaline urine (target pH > 7.5) reduces precipitation of calcium oxalate crystals in tubules and enhances renal excretion - **Haemodialysis:** Removes ethylene glycol, glycolic acid, and oxalic acid; corrects severe acidosis; manages fluid overload and hyperkalaemia **Warning:** Delaying fomepizole and bicarbonate to initiate dialysis alone is suboptimal — both should be started immediately in parallel.