Final answer:
A nurse should expect to first observe a skin rash as an initial sign of an anaphylactic reaction after administering an IM antibiotic to a child, as it typically precedes other severe symptoms of anaphylaxis.
Step-by-step explanation:
A nurse observing a child for signs of anaphylactic reaction after administering an IM antibiotic would expect certain manifestations based on the systemic allergic response known as anaphylaxis. The initial symptoms of such a reaction can include skin rash, wheezing, hypotension, and nausea and vomiting. However, the manifestation that a nurse should expect to observe first is typically skin rash (option A), which can occur due to the release of histamines causing a wheal and flare response in the skin. Immediate medical attention with epinephrine is crucial for treating anaphylactic shock to counteract the serious symptoms like plummeting blood pressure, respiratory distress, and potential airway blockage.