What is the first step in treating a child with suspected anaphylaxis?

The correct answer is C: Give epinephrine. Administering epinephrine is the first and most critical step in treating anaphylaxis. Epinephrine rapidly reverses the symptoms of anaphylaxis, including airway swelling, hypotension, and shock. Delaying administration can lead to severe complications or death, making it essential in emergency treatment. Choice A, administering oxygen, might be necessary but should not delay the administration of epinephrine. Starting an IV line (Choice B) is important for further treatment but not the initial step. Monitoring vital signs (Choice D) is essential but comes after administering epinephrine to stabilize the child.