Which option correctly differentiates in vivo and in vitro bioassay methods?

• A. In vivo measures inside the body (whole-body or organ counting); in vitro analyzes excreta (urine/feces) for radionuclides.
• B. In vivo analyzes excreta; in vitro measures whole-body counting.
• C. In vivo uses air samples; in vitro uses water samples.
• D. In vivo is always more accurate than in vitro.

Answer: (A)

Measuring internal radioactivity can be done directly inside the body or indirectly by testing samples taken from the body. In vivo bioassays use detectors placed on or around the body to measure radioactivity that remains inside, such as whole-body counting or organ-specific counting. In vitro bioassays analyze biological samples removed from the body, typically urine or feces, to detect radionuclides excreted by the body and often use those results with biokinetic models to estimate the internal burden. This distinction—direct internal measurement versus analysis of excreted material—is what the correct option describes.