Kaplan USMLE Step 3: Exposure to an unknown substance brings boy to ED

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

A 10-year-old boy is brought to the emergency department by his parents after he sprayed himself while playing with an unmarked spray can. After several minutes, he began tearing and drooling according to his mother. By the time his father arrived, the boy had collapsed on the floor and was unresponsive. On examination, the boy appears comatose and is profusely diaphoretic. His pulse is 48/min and his blood pressure is 80/40 mm Hg. The patient's respirations are slow and shallow, and his expiratory time is prolonged. Pupils are constricted and unresponsive. Lung auscultation reveals bilateral wheezes. Multiple fasciculations are observed in various muscle groups. Pulse oximetry measures an oxygen saturation of 70% on room air. An arterial blood gas shows:

• pH: 7.05
• pCO₂: 83 mmHg
• pO₂: 44 mmHg

A chest radiograph shows clear lungs. The child is immediately intubated and ventilated on 100% supplemental oxygen. Intravenous saline infusion is started. Which of the following is the most appropriate next step in management?

A. Administer atropine sulfate intravenously

B. Administer corticosteroids intravenously

C. Administer edrophonium intravenously

D. Administer epinephrine subcutaneously

E. Administer methylene blue intravenously

The correct answer is A.

The child is exhibiting symptoms of excessive parasympathetic tone after exposure to an unknown substance. This is a classic case of severe organophosphate poisoning. Symptoms include lacrimation, salivation, blurred vision, urinary incontinence, diarrhea and diaphoresis. Other more severe effects are bronchospasm, bradycardia, hypotension, respiratory paralysis and mental status change.

Organophosphates include many pesticides that irreversibly react with postsynaptic acetylcholinesterase molecules and inhibit their activity. Atropine is the drug of choice to treat the acute toxic effects of organophosphate poisoning by blocking muscarinic acetylcholine receptors. In addition, an antidote, pralidoxime, can reactivate acetylcholinesterases bound by organophosphates and should be administered along with atropine.

Choice B. The bronchospasm and hypotension arises from increased cholinergic activity not from an inflammatory or anaphylactic response. Corticosteroids are not useful in this situation.

Choice C. Edrophonium is an inhibitor of acetylcholinesterase and has effects similar to organophosphates, so it would only worsen his symptoms.