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

An uptake probe is specifically designed to measure the amount of radioactive material that is taken up by certain organs or tissues in the body, which is a crucial aspect of nuclear medicine imaging. In the context of studies related to the spleen, the uptake probe is particularly useful for examining how well the spleen is functioning in terms of its ability to sequester or trap radiolabeled red blood cells.

In splenic sequestration studies, the uptake probe can assess the extent of blood cell retention in the spleen, thus providing valuable insights into splenic function and any potential abnormalities. This is particularly significant in determining conditions such as hypersplenism or other disorders affecting the spleen's ability to filter blood.

On the contrary, the other options focus on different parameters related to blood volume and cell characteristics, which typically involve different techniques or methods, such as blood sampling, rather than using the uptake probe specifically for organ functionality.