Which statement best distinguishes external exposure from internal exposure in health physics?

• A. External exposure comes from sources outside the body; internal exposure comes from radionuclides taken into the body and distributed internally.
• B. External exposure comes from inside the body; internal exposure comes from external sources.
• C. External exposure is the same as internal exposure.
• D. Internal exposure only occurs with ingested materials; external exposure only with external radiation.

Answer: (A)

The main idea being tested is how exposure is categorized by where the radioactive source is relative to the body. External exposure comes from radiation emitted by sources outside the body and striking tissues from the outside. Internal exposure arises when radionuclides enter the body—through inhalation, ingestion, or absorption—and then emit radiation from inside, delivering dose from within.

This is why the statement identifying the distinction is best: external exposure comes from sources outside the body, while internal exposure comes from radionuclides taken into the body and distributed internally. Internal dose can come from inhaled or ingested materials (and even materials that enter through wounds), and external exposure includes radiation from sources outside the body irradiating it from the outside.

The other notions are incomplete or incorrect because they either flip the sources, claim they are the same, or overly restrict internal exposure to ingestion and external exposure to external radiation.