Which of the following defines a preventive service?

A preventive service is one that is specifically designed to prevent diseases and maintain health. These services focus on reducing the risk of developing health problems before they occur. Examples of preventive services include vaccinations, annual physical exams, screenings for various conditions, and counseling on lifestyle choices such as diet and exercise. The goal of these services is to catch potential health issues early on or even prevent them from developing, thus promoting overall wellness.

In contrast, other services mentioned in the options pertain to different healthcare paradigms. Treatment provided after a medical issue arises focuses on managing and treating existing conditions rather than preventing them. Urgent care services correspond to immediate treatment required for sudden illness or injury, which is inherently reactive. Finally, diagnostic tests for existing conditions aim to identify or assess already known health issues rather than prevent them. Therefore, option A clearly defines the essence of a preventive service within the healthcare system.