Final answer:
Ultrasound is the safest test to detect kidney tumors or cysts as it uses sound waves and does not expose the patient to ionizing radiation, making it particularly safe during pregnancy.
Step-by-step explanation:
The safest test to detect kidney tumors or cysts, especially during pregnancy or when the patient requires a non-invasive and radiation-free procedure, is an ultrasound. Unlike CT scans, PET scans, and X-rays, which use ionizing radiation and could potentially pose risks, an ultrasound uses sound waves to create images and carries no risk of radiation. Additionally, MRI is considered safe as it uses magnetic fields and radio waves, but between the two, ultrasound is more commonly used during pregnancy due to its safety profile and convenience.