What medical term refers to an instrument used to examine the inside of a body cavity?

The term that refers to an instrument used to examine the inside of a body cavity is "endoscope." This device is specifically designed to enter various body cavities, enabling healthcare professionals to visualize internal structures and diagnose conditions. Endoscopes come in various forms depending on the area being examined; for instance, a gastroscope is used for the stomach, while a colonoscope is used for the colon.

In contrast, ultrasound is a diagnostic imaging technique that uses sound waves to visualize soft tissues and organs, but it does not involve direct examination of internal cavities. A stethoscope is primarily used for listening to heartbeats, lung sounds, and other bodily noises, without providing visual access to internal structures. An otoscope is a specialized tool for examining the ear canal and tympanic membrane, but its use is limited to that specific area rather than examining various body cavities. Thus, the correct choice accurately reflects the function of the instrument described in the question.