Which statement about StructureDefinition is true?

• A. It lists all resources and defines constraints
• B. It describes the interfaces for data exchange
• C. It is a serialization format
• D. It constrains the optionality and cardinality of elements

Answer: (D)

StructureDefinition is the mechanism for describing how a resource or data type is organized and what constraints apply to its fields. The essential aspect it encodes is how many times an element can appear and whether it must be present, which is the concept of optionality and cardinality. In a StructureDefinition, each element has constraints like a minimum and maximum occurrence, which directly control whether a field is required or optional and how many times it can repeat. This is how a profile or defined data type enforces additional rules beyond the base resource.

So the statement that StructureDefinition constrains the optionality and cardinality of elements is the best fit. It isn’t about listing all resources, describing interfaces for data exchange, or serving as a serialization format—the other options describe different parts of FHIR or data handling, not the role of StructureDefinition.