A nurse is mixing a short-acting insulin and an intermediate-acting insulin in the same syringe for a client who has diabetes mellitus. Which of the following actions should the nurse take first?

The correct answer is C: Inject air into the intermediate-acting insulin vial. This should be done first to prevent contamination of the insulin vials. By injecting air into the intermediate-acting insulin vial first, the nurse can easily withdraw the correct dose without causing any risk of contamination. Drawing the short-acting insulin first or injecting air into the short-acting insulin vial could lead to mixing up the insulins or contaminating the vials. Drawing the intermediate-acting insulin first without injecting air could create a vacuum in the vial, making it difficult to withdraw the correct dose. Therefore, injecting air into the intermediate-acting insulin vial is the crucial first step in preparing a mixed insulin injection.