Final answer:
A pacemaker prevents a patient from having an MRI because the device contains metal and electronic components that can be affected by the strong magnetic field.
Step-by-step explanation:
The correct answer is A) She has a pacemaker. A pacemaker is a device that helps regulate the heart's rhythm. It contains metal and electronic components that can be affected by the strong magnetic field of an MRI machine. This can cause the pacemaker to malfunction or move, posing a risk to the patient's health. Therefore, individuals with pacemakers are not eligible for an MRI scan.