A 29-year-old artisan jeweler is brought to the emergency department by his roommate after… — USMLE Step 2 CK

A 29-year-old artisan jeweler is brought to the emergency department by his roommate after being found unresponsive in his basement workshop. The workshop is poorly ventilated and contains open vats of chemical solutions used for metal extraction and electroplating. On physical examination, his temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $85/50\text{ mm Hg}$ , heart rate is $132/\text{min}$ , and respiratory rate is $28/\text{min}$ . Pulse oximetry shows $99\%$ oxygen saturation on room air. His skin appears flushed and abnormally pink.

Laboratory evaluation reveals the following:

Test	Value
Serum sodium	$140\text{ mEq/L}$
Serum potassium	$4.5\text{ mEq/L}$
Serum chloride	$100\text{ mEq/L}$
Serum bicarbonate	$8\text{ mEq/L}$
Blood urea nitrogen	$15\text{ mg/dL}$
Serum creatinine	$1.0\text{ mg/dL}$
Serum lactate	$12\text{ mmol/L}$ (normal: $0.5 - 1.5\text{ mmol/L}$ )

Arterial blood gas analysis on room air reveals:

Test	Value
pH	$7.21$
pCO2	$20\text{ mm Hg}$
pO2	$95\text{ mm Hg}$

An intravenous antidote regimen including sodium nitrite is immediately administered. Which of the following statements regarding this patient's condition, acute management, or subsequent occupational investigation is most accurate?

A
The arterial blood gas demonstrates a mixed high anion gap metabolic acidosis and primary respiratory alkalosis, necessitating evaluation for a different toxicologic ingestion.
B
If an occupational screening test for chronic exposure to this chemical is deployed in a facility with a much lower prevalence, the negative predictive value of the test will decrease.
The administered antidote acts by oxidizing hemoglobin to a ferric state, which subsequently sequesters the toxic compound and allows restoration of cellular respiration.Answer
D
Subsequent epidemiological investigation of this workplace hazard should utilize a retrospective cohort design by selecting other jewelers based on whether they have developed toxicity symptoms.
E
Following the patient's stabilization, occupational health authorities should primarily focus on punitive disciplinary measures against the worker for failing to use adequate ventilation.

Answer

The administered antidote acts by oxidizing hemoglobin to a ferric state, which subsequently sequesters the toxic compound and allows restoration of cellular respiration.

The correct answer accurately describes the mechanism of sodium nitrite, a key component in the traditional cyanide antidote kit. This patient's occupational history of electroplating, combined with severe lactic acidosis, abnormally pink skin (due to decreased tissue oxygen extraction), and normal arterial oxygenation, strongly points to acute cyanide poisoning. Cyanide binds to the ferric iron (

Fe^{3+}

) in cytochrome c oxidase (Complex IV), halting oxidative phosphorylation and forcing the body into anaerobic metabolism (causing lactic acidosis). Sodium nitrite acts by oxidizing the ferrous iron (

Fe^{2+}

) in normal hemoglobin to the ferric state (

Fe^{3+}

), creating methemoglobin. Methemoglobin has a massive affinity for cyanide and acts as a sink, drawing cyanide away from the mitochondria to form cyanmethemoglobin, thereby restoring cellular respiration.

Step-by-Step Solution

Identify the occupational hazard and clinical toxidrome.

The patient's work in electroplating, poorly ventilated environment, flushed pink skin, severe lactic acidosis, and normal arterial pO2 point directly to acute cyanide poisoning.

Cyanide blocks oxidative phosphorylation, leading to severe lactic acidosis while preventing tissues from extracting oxygen from the blood (causing pink skin and normal pO2).

Analyze the acid-base status using the provided laboratory data.

The anion gap is calculated as

140 - (100 + 8) = 32

, confirming a high anion gap metabolic acidosis. Applying Winter's formula (

1.5 \times 8 + 8 = 20\text{ mm Hg}

) matches the measured

pCO_2

20\text{ mm Hg}

Confirming pure respiratory compensation rules out mixed acid-base disorders like concurrent primary respiratory alkalosis, which would suggest a different toxidrome (e.g., salicylate poisoning).

Determine the pharmacological mechanism of the administered antidote.

Sodium nitrite is administered to intentionally oxidize normal ferrous hemoglobin (

Fe^{2+}

) to ferric methemoglobin (

Fe^{3+}

Methemoglobin has a high affinity for cyanide, pulling it away from mitochondrial cytochrome c oxidase (Complex IV) and allowing aerobic cellular respiration to resume.

Evaluate the distractors for conceptual accuracy.

The distractors contain fundamental errors in acid-base physiology, biostatistics (predictive values, study design), and medical ethics (blame vs. root cause analysis).

Only the option describing the oxidation of hemoglobin correctly aligns with the medical facts of the case and sound pathophysiological principles.

Key Concept

Cyanide toxicity presents with severe lactic acidosis and normal arterial oxygen tension; treatment involves sodium nitrite, which induces methemoglobinemia to sequester the cyanide.

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