An uptake probe would be used for which of the following studies?

An uptake probe is primarily utilized to measure the uptake of radiopharmaceuticals by specific organs, commonly employed in studies assessing thyroid function. In the context of the choices provided, the study most closely associated with the use of an uptake probe is splenic sequestration.

When evaluating splenic function, measuring the uptake of radiolabeled blood elements or other tracers can provide insights into how effectively the spleen is filtering and sequestering components of blood, which is essential for diagnosing conditions related to splenic or hematological disorders.

The other options involve specific blood component evaluations. Red cell mass and red cell survival studies typically require radiolabeled red blood cells and would use techniques such as blood sampling and radioactive decay measurements rather than an uptake probe. Plasma volume tests similarly do not rely on an uptake probe but rather on other methodologies, such as injecting a radiolabeled tracer and calculating the volume based on the dilution principle.

Thus, the use of an uptake probe is indeed most appropriate for splenic sequestration assessments, allowing for direct measurement of radiopharmaceutical uptake related to spleen functionality.