FPGEE Toxicology 3 — Questions and Answers
Question 1: A patient ingests methanol and presents with an elevated anion gap, elevated osmol gap, and visual disturbances. What toxic metabolite causes the ocular damage?
- Methanol itself
- Formic acid (Correct answer)
- Oxalic acid
- Acetaldehyde
Correct answer: Formic acid
Methanol is metabolized by alcohol dehydrogenase to formaldehyde and then formic acid, which inhibits cytochrome oxidase and causes optic nerve toxicity.
Question 2: Fomepizole (4-MP) is used to treat methanol and ethylene glycol poisoning because it:
- Enhances renal excretion of glycolate
- Inhibits alcohol dehydrogenase (Correct answer)
- Provides competitive substrate for aldehyde dehydrogenase
- Chelates oxalate crystals
Correct answer: Inhibits alcohol dehydrogenase
Fomepizole is a potent inhibitor of alcohol dehydrogenase, preventing conversion of methanol and ethylene glycol to their toxic metabolites.
Question 3: Which of the following best describes the mechanism of toxicity of carbon monoxide?
- It displaces oxygen by binding hemoglobin with higher affinity and inhibits cellular respiration (Correct answer)
- It oxidizes hemoglobin iron to the ferric state
- It depletes glutathione in hepatocytes
- It competitively inhibits GABA receptors
Correct answer: It displaces oxygen by binding hemoglobin with higher affinity and inhibits cellular respiration
CO binds hemoglobin with 250 times the affinity of oxygen forming carboxyhemoglobin and also inhibits cytochrome oxidase, causing both impaired oxygen delivery and cellular hypoxia.
Question 4: A patient with salicylate toxicity has a blood pH of 7.5 and urine pH of 6.5. What urinary manipulation should be performed to enhance elimination?
- Acidify the urine with ammonium chloride
- Alkalize the urine with sodium bicarbonate (Correct answer)
- Administer mannitol for osmotic diuresis
- No urinary manipulation is needed
Correct answer: Alkalize the urine with sodium bicarbonate
Urinary alkalinization with sodium bicarbonate raises urine pH, trapping ionized salicylate in the tubular lumen and dramatically increasing renal clearance.
Question 5: Which opioid has a significantly longer duration of action than naloxone, requiring repeated dosing or a naloxone infusion?
- Fentanyl
- Methadone (Correct answer)
- Codeine
- Hydromorphone
Correct answer: Methadone
Methadone has a half-life of 24-60 hours, far exceeding naloxone's 60-90 minutes, necessitating repeat doses or a continuous naloxone infusion to prevent re-narcotization.
Question 6: In acute digoxin toxicity, which electrolyte abnormality is most likely to potentiate toxicity and worsen arrhythmias?
- Hypernatremia
- Hypokalemia (Correct answer)
- Hypermagnesemia
- Hypocalcemia
Correct answer: Hypokalemia
Hypokalemia increases digoxin binding to Na+/K+-ATPase because potassium and digoxin compete for the same binding site, potentiating toxicity.
Question 7: Digoxin-specific antibody fragments (Fab) work by:
- Increasing renal clearance of digoxin via active secretion
- Binding free digoxin and removing it from receptor sites (Correct answer)
- Competitively blocking cardiac Na+/K+-ATPase
- Inducing hepatic metabolism of digoxin
Correct answer: Binding free digoxin and removing it from receptor sites
Digoxin immune Fab fragments bind free digoxin molecules with high affinity, forming complexes that are renally excreted, removing digoxin from its toxic sites of action.
FPGEE Toxicology 3
A patient ingests methanol and presents with an elevated anion gap, elevated osmol gap, and visual disturbances.
What toxic metabolite causes the ocular damage?