Final answer:
MRA is a medical imaging technique that is used to detect blood flow in the body, making option (a) the correct answer.
Step-by-step explanation:
Magnetic Resonance Angiography (MRA) is a type of medical imaging technique used to detect blood flow in the body. MRA utilizes a magnetic field and pulses of radio wave energy to provide pictures of blood vessels inside the body. It's particularly useful for assessing blood flow to the heart muscle and can help detect conditions that affect the blood vessels.
As for the options given in the question, MRA is not typically used to detect pathogens, tumor cells, or stones in the bladder. Other medical imaging techniques such as ultrasound are used to monitor the health of a pregnancy, and CT scans can show detailed images of the body which can help detect tumors. If there is a need to diagnose conditions within the bladder, such as stones, a combination of an ultrasound and a urine culture might be utilized for a more accurate diagnosis.