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Difficulty: HardCellular Injury and Adaptation

A 3232-year-old industrial worker is brought to the emergency department after losing consciousness in a metal plating facility. Coworkers report a chemical spill occurred shortly before he collapsed. On examination, he is comatose with a respiratory rate of 2828/min and a heart rate of 120120/min. His skin appears flushed. A faint "bitter almond" odor is noted on his clothing. Arterial blood gas analysis on room air reveals:

ParameterPatient ValueReference Range
pH7.127.127.35−7.457.35 - 7.45
pCO2_22424 mmHg35−4535 - 45 mmHg
pO2_29898 mmHg80−10080 - 100 mmHg
HCO3−_3^-88 mEq/L22−2822 - 28 mEq/L
Lactate14.514.5 mmol/L0.5−1.00.5 - 1.0 mmol/L

Which of the following describes the primary subcellular mechanism responsible for this patient's profound cellular injury?

  1. A
    Competitive binding to hemoglobin with an affinity significantly greater than oxygen, severely reducing oxygen-carrying capacity.
  2. B
    Increased permeability of the inner mitochondrial membrane to protons, uncoupling oxidative phosphorylation.
  3. C
    Systemic activation of the coagulation cascade resulting in microvascular thrombosis and impaired tissue perfusion.
  4. Inhibition of mitochondrial cytochrome c oxidase by binding to its ferric iron component, preventing oxygen reduction.Answer
  5. E
    Direct inhibition of the plasma membrane Na +^+/K +^+ ATPase, leading to cellular swelling and oncotic necrosis.

Answer

Inhibition of mitochondrial cytochrome c oxidase by binding to its ferric iron component, preventing oxygen reduction.
The clinical presentation of rapid-onset coma, severe lactic acidosis, normal arterial oxygen tension, and a 'bitter almond' odor in a metal plating worker is pathognomonic for acute cyanide poisoning. Cyanide induces cellular injury by acting as a chemical asphyxiant. It rapidly crosses cell membranes and binds with high affinity to the ferric iron (Fe3+) within cytochrome c oxidase (Complex IV) of the mitochondrial electron transport chain. This binding completely blocks the transfer of electrons to oxygen, halting oxidative phosphorylation and ATP production. Consequently, cells are forced into anaerobic metabolism, generating massive amounts of lactate. Because tissues cannot extract oxygen from the blood, venous blood remains highly oxygenated, which clinically manifests as flushed skin.

Step-by-Step Solution

1
Analyze the clinical presentation and occupational history.
The patient is a metal plating worker presenting with sudden collapse, flushed skin, and a bitter almond odor, which strongly suggests acute chemical intoxication.
Metal plating is a high-risk occupation for cyanide exposure, and the bitter almond odor is a classic (though inconsistently detected) finding.
2
Interpret the laboratory findings.
The ABG shows severe high anion gap metabolic acidosis with a normal pO2 and markedly elevated lactate.
This profile indicates profound tissue hypoxia and reliance on anaerobic metabolism despite adequate oxygen delivery to the blood, pointing to chemical asphyxiation.
3
Identify the underlying toxicologic cellular mechanism.
Cyanide binds to the ferric iron (Fe3+) in cytochrome c oxidase (Complex IV) of the mitochondrial electron transport chain.
This interaction halts oxidative phosphorylation, preventing oxygen utilization and leading to rapid ATP depletion, cellular asphyxiation, and lactic acidosis.

Key Concept

Cyanide induces severe cellular injury by binding to the ferric iron (Fe3+) in cytochrome c oxidase (Complex IV), completely halting aerobic metabolism and ATP production.
Estimated Time:2m 0s
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Topics

Cellular Injury and Adaptation

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