What is the most likely cause of a young adult's presentation with difficulty arousing, rapid breathing, tachycardia, alkalosis, elevated liver enzymes, and impaired renal function after an emotionally distressing event, with no significant prior medical history or medication use?

Salicylate Toxicity (Aspirin Overdose)

This patient's presentation is most consistent with acute salicylate (aspirin) toxicity, which requires immediate treatment with IV sodium bicarbonate, aggressive fluid resuscitation, and consideration of hemodialysis.

Clinical Reasoning

The constellation of findings points definitively toward salicylate poisoning:

• Respiratory alkalosis with tachypnea: The pH of 7.48 with PCO2 of 18 indicates primary respiratory alkalosis from direct stimulation of the respiratory center by salicylates 1
• Altered mental status: Difficulty arousing suggests CNS toxicity from salicylate accumulation 2
• Tachycardia (HR 135): Consistent with sympathetic activation and metabolic stress 1
• Acute kidney injury: BUN 22, creatinine 1.5 indicating renal dysfunction 2
• Elevated transaminases: ALT 60, AST 54 suggesting hepatic toxicity 2
• Emotional distress preceding presentation: Young adults may impulsively ingest large quantities of over-the-counter medications (aspirin) following relationship conflicts 3

Critical Diagnostic Steps

Immediately obtain:

• Serum salicylate level (therapeutic range 10-30 mg/dL; toxic >30 mg/dL; severe >100 mg/dL)
• Complete metabolic panel including anion gap calculation
• Arterial blood gas (already shows respiratory alkalosis, but may evolve to mixed disorder)
• Urine pH
• Acetaminophen level (co-ingestion common)

The respiratory alkalosis will typically progress to a mixed respiratory alkalosis/metabolic acidosis as salicylate toxicity worsens, creating an elevated anion gap 1.

Immediate Management Algorithm

First-line treatment:

1. Airway protection: Given altered mental status, assess need for intubation carefully—avoid if possible as mechanical ventilation can worsen acidosis 3
2. IV sodium bicarbonate: Start 150 mEq in 1L D5W at 150-200 mL/hr to alkalinize urine (target urine pH >7.5) and trap ionized salicylate in urine 3
3. Aggressive IV hydration: 2-3L crystalloid bolus to restore renal perfusion 2
4. Potassium repletion: Essential for effective urinary alkalinization (hypokalemia prevents bicarbonate excretion)

Second-line treatment:

• Hemodialysis indications: Salicylate level >100 mg/dL, severe acidosis (pH <7.2), renal failure, pulmonary edema, altered mental status refractory to initial therapy, or clinical deterioration 3

Key Pitfalls to Avoid

• Do NOT intubate unless absolutely necessary: Mechanical ventilation reduces minute ventilation and can precipitate severe metabolic acidosis and cardiovascular collapse in salicylate toxicity 3
• Do NOT give activated charcoal if patient cannot protect airway: Risk of aspiration pneumonia outweighs benefit 3
• Do NOT delay bicarbonate therapy: Urinary alkalinization is the cornerstone of treatment and should begin immediately 3

Differential Considerations

While the presentation could theoretically suggest:

• Serotonin syndrome: Would require recent serotonergic drug exposure and typically presents with clonus, hyperreflexia, and hyperthermia (temperature here is 99.4°F, essentially normal) 3
• Hyperventilation syndrome: Could explain respiratory alkalosis but would not cause altered mental status, renal dysfunction, or elevated transaminases 4
• Sepsis: Could cause tachycardia and organ dysfunction but would typically present with metabolic acidosis, not alkalosis 3

The combination of respiratory alkalosis, altered mental status, multi-organ dysfunction (hepatic, renal), and preceding emotional distress in a young person with no medical history makes salicylate toxicity the unifying diagnosis 2, 1.