Which of the following is an example of a first-dose reaction?

The example of a first-dose reaction is orthostatic hypotension. This condition can occur following the initial administration of certain medications, especially those that affect the cardiovascular system. When a patient first takes a medication that causes vasodilation or reduces blood volume, they may experience a sudden drop in blood pressure upon standing, resulting in dizziness or fainting. This phenomenon emphasizes the importance of monitoring patients closely after the first dose of medications like antihypertensives, which are commonly associated with first-dose hypotension.

The other options describe reactions that are not specifically related to the first dose of a medication. Purple glove syndrome from phenytoin is related to intravenous administration of the drug and doesn't occur with the first dose in all patients. Hemolytic anemia from ceftriaxone can develop over time due to an immune reaction but is not classified as a first-dose reaction. Contact dermatitis from neomycin arises from prolonged exposure or sensitivity rather than a first encounter. Therefore, orthostatic hypotension stands out as the appropriate example of a reaction that can occur with the very first dose of a particular medication.