**Core Concept**
A goniometer is a medical device used to measure the angle between two planes or surfaces in the body, typically in the context of joint mobility or spinal alignment. This is crucial in assessing the range of motion and detecting any abnormalities that may indicate underlying conditions such as arthritis, osteoporosis, or spinal deformities.

**Why the Correct Answer is Right**
The goniometer is employed to evaluate joint mobility by measuring the angle of movement in degrees. This measurement is essential in assessing the functional capabilities of a patient, especially in the context of rehabilitation or post-surgical care. The device is typically used in conjunction with other assessment tools to provide a comprehensive evaluation of joint function.

**Why Each Wrong Option is Incorrect**
**Option A:** A goniometer is not used for measuring blood pressure, as this is the primary function of a sphygmomanometer.
**Option B:** While a goniometer may be used in conjunction with other equipment during a physical examination, it is not a diagnostic tool for imaging studies such as X-rays or MRIs.
**Option C:** A goniometer is not a device used for measuring temperature, which is typically done with a thermometer.

**Clinical Pearl / High-Yield Fact**
When using a goniometer, it's essential to ensure that the device is properly aligned with the joint axis to obtain accurate measurements. Misalignment can lead to incorrect assessments and potential misdiagnosis.

**Correct Answer: D. Measuring joint mobility.**