Which of the following is the primary cause of this patient's acid-base disturbance?

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Quizplus · Accepted Answer

Cyanide is a potent inhibitor of cytochrome oxidase a3 in the mitochondrial electron transport chain. It binds to ferric iron (Fe3+), inhibiting its reduction to ferrous iron (Fe2+) and blocking production of ATP from oxidative phosphorylation. Cells then switch to anaerobic metabolism, leading to lactic acid formation and causing metabolic acidosis. Serum bicarbonate levels fall in an attempt to buffer excess acid. Metabolic acidosis also triggers central and peripheral chemoreceptors, increasing alveolar ventilation and then presenting as tachypnea. This leads to a fall in arterial PCO₂ (PaCO2). (Choice A) Impaired excretion of lactic acid and other serum acids occurs in renal failure and results in anion gap metabolic acidosis. (Choice B) Increased gut absorption of lactic acid may occur in blind loop syndrome, in which excessive production and absorption of D-lactate causes hypochloremic metabolic acidosis. (Choice C) Increased ketogenesis occurs during starvation and with lack of insulin in insulin dependent diabetics. (Choice D) An increased metabolic rate may cause lactic acidosis if insufficient oxygen is present in tissues and anaerobic metabolism ensues. It is common in post-seizure lactic acidosis. Educational objective: Cyanide binds to ferric iron in cytochrome oxidase a3 in the mitochondrial electron transport chain. This activity blocks oxidative phosphorylation and promotes anaerobic metabolism, causing lactic acidosis. References: Carbon monoxide and cyanide toxicity: etiology, pathophysiology and treatment in inhalation injury. (http://www.ncbi.nlm.nih.gov/pubmed/23547992)

Which of the Following Is the Primary Cause of This