Which of the following statements about the three-step extension process is correct?

• A. Use extension; Define extension; Register extension
• B. Register extension; Use extension; Define extension
• C. Define extension; Use extension; Register extension
• D. Define extension; Register extension; Use extension

Answer: (D)

In this context, the process of using a FHIR extension follows a clear order: first define its structure and semantics, then register it so its identity and description are publicly documented, and finally use it in resources by attaching the extension data via its URL. Defining the extension sets what data it carries and how it should be interpreted. Registering it makes the extension discoverable and unambiguous for all systems, providing a canonical reference that tooling can rely on. Only after these steps is it appropriate to apply the extension to real resources, using the defined URL to attach the extension data. Skipping the define step or registering after attempting to use would leave consumers without a known structure or a discoverable identity, making interoperability unstable.