Final answer:
The measurement of a prospective blood donor's blood pressure is a key indicator of their eligibility to donate blood, as it provides an essential insight into the donor's health status.
Step-by-step explanation:
A phlebotomy technician performing a brief physical examination on a prospective blood donor would be interested in checking measurements that indicate the donor’s eligibility to donate blood. While several parameters are checked, one key measurement is the donor’s blood pressure, which is a common medical examination and a valuable indicator of an individual's health status. Other eligibility criteria for donating blood relate to the donor’s overall health condition and may include temperature, weight, and often an assessment to ensure they do not have particular risk factors or diseases that would make donating blood unsafe for the recipient.