Which statement correctly distinguishes external exposure from internal exposure?

• A. External exposure occurs from inside the body; internal exposure from outside.
• B. External exposure and internal exposure are measured by the same method.
• C. Internal exposure cannot be measured.
• D. External exposure is irradiation from sources outside the body; internal exposure results from radionuclides taken into the body via inhalation or ingestion.

Answer: (D)

The key idea is the source and route of radiation causing the dose. External exposure comes from radiation sources outside the body, so the energy deposited in tissues occurs as radiation travels through the body from an external source. Internal exposure happens when radionuclides are inside the body, having been taken in by inhalation or ingestion, and these internal emitters deliver dose directly from within tissues over time.

This makes the described statement the best choice: external exposure is irradiation from sources outside the body; internal exposure results from radionuclides taken into the body via inhalation or ingestion. External exposure is typically assessed with dosimeters worn on the body to measure dose from outside, while internal exposure requires internal dosimetry methods—such as whole-body counting, organ counting, or bioassays—to estimate the amount of radioactivity inside the body and the dose to organs based on biokinetics and retention.