**Core Concept**
A goniometer is a medical instrument used to measure the angle between two planes or the degree of movement at a joint. It is essential in assessing joint mobility, range of motion, and detecting any abnormalities in the musculoskeletal system.

**Why the Correct Answer is Right**
A goniometer is used to quantify joint mobility and detect any limitations in movement. This is particularly useful in assessing patients with musculoskeletal disorders, such as arthritis, or after joint injuries or surgeries. By measuring the degree of movement, healthcare professionals can monitor the progression of the condition, assess the effectiveness of treatment, and make informed decisions about rehabilitation or further interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** A stethoscope is used to listen to internal sounds of the body, such as heart and lung sounds, and is not used to measure joint mobility.
**Option B:** A sphygmomanometer is used to measure blood pressure and is not related to joint mobility.
**Option C:** A caliper is used to measure the size or thickness of an object, but not the angle or degree of movement at a joint.

**Clinical Pearl / High-Yield Fact**
When using a goniometer, it's essential to ensure that the measurement is taken at the same point on the joint on both sides of the body to obtain accurate results. This can be achieved by marking the point of measurement with a marker or using a standardized technique.

**Correct Answer: C. A caliper is used to measure the size or thickness of an object, but not the angle or degree of movement at a joint.**