Which term would use the prefix "masto-"?

The prefix "masto-" specifically refers to the breast. In medical terminology, the term "mastectomy" is the surgical procedure for the removal of one or both breasts, typically performed as a treatment for breast cancer or other breast diseases. The use of "masto-" in this context indicates a direct relation to breast tissue.

In contrast, the other choices involve prefixes that pertain to different body organs or systems: "nephrology" involves the kidneys (from "nephro-"), "cardiology" relates to the heart (from "cardio-"), and "orchiectomy" refers to the removal of testicles (from "orchido-"). Thus, the term "mastectomy" is the appropriate choice as it correctly utilizes the "masto-" prefix associated with breast-related medical procedures.