What is the term for a procedure that uses a long, flexible tube inserted into a patient?

The procedure that involves using a long, flexible tube inserted into a patient is known as endoscopy. This technique allows healthcare professionals to visualize the interior of a hollow organ or cavity within the body. The tube, commonly equipped with a camera and light source, enables direct observation and can be used for diagnostic purposes, such as examining the gastrointestinal tract, or for therapeutic interventions, such as removing polyps or taking biopsies.

In contrast, ultrasound imaging employs sound waves to produce images of internal structures, but it does not involve the insertion of a tube. MRI (Magnetic Resonance Imaging) and CT scans (Computed Tomography) are imaging techniques that rely on magnetic fields and X-rays, respectively, to create detailed images of the body, but they also do not use a flexible tube for visualization. Thus, endoscopy is the procedure that specifically aligns with the description given in the question.