The radionuclidic purity decreases because of the longer physical half-life of which radionuclide compared to 99mTc?

The correct choice regarding the decrease in radionuclidic purity related to a radionuclide with a longer physical half-life compared to technetium-99m (99mTc) is indeed the one associated with 99Mo, or molybdenum-99.

99mTc is derived from 99Mo through a decay process, where 99Mo has a physical half-life of about 66 hours. This long half-life means that if 99Mo is present during the production of 99mTc, it will not decay rapidly and can continue to be present alongside the desired 99mTc, thus affecting the overall purity of the radionuclide mixture.

Radionuclidic purity is essential to ensure that the administered dose has the correct composition for diagnostic or therapeutic procedures. If a longer-lived radionuclide like 99Mo is present in significant quantities, it can compromise the purity by increasing the non-target radionuclide content in the sample.

In contrast, the other options listed, such as 99Ir (iridium-99), 125I (iodine-125), and 131I (iodine-131), have different half-lives and properties that do not directly impact the purity