Final answer:
Medical assistants are required to wear a dosimeter when using an X-ray machine to measure exposure to ionizing radiation for safety purposes.
Step-by-step explanation:
Safety regulations require a medical assistant to wear a dosimeter when using an X-ray machine. Dosimeters are devices used to measure exposure to ionizing radiation, which is a type of radiation emitted by X-ray machines. Given that X-rays can pose potential health risks due to prolonged exposure, it's crucial for medical personnel to monitor their radiation levels to ensure safety. Other equipment such as blood pressure cuffs, autoclaves, and EKG machines do not emit ionizing radiation and therefore do not require the use of a dosimeter.