**Core Concept**
Ultrasound is a non-invasive imaging modality that uses high-frequency sound waves to produce images of internal structures. It is widely used in various medical specialties for diagnostic purposes, including obstetrics, cardiology, and abdominal imaging.

**Why the Correct Answer is Right**
Ultrasound is an effective tool for diagnosing conditions such as gallstones, liver disease, and fetal development. It works by emitting sound waves that bounce off internal structures, producing echoes that are converted into images. The technology is particularly useful for visualizing soft tissues and fluid-filled structures.

**Why Each Wrong Option is Incorrect**
**Option A:** While ultrasound can be used to evaluate joint effusions, it is not the primary modality for diagnosing conditions such as osteoarthritis or rheumatoid arthritis.
**Option B:** Ultrasound can be used to guide biopsies and injections, but it is not typically used for diagnosing conditions such as multiple sclerosis.
**Option C:** Ultrasound is not typically used for diagnosing conditions such as pulmonary embolism, which is usually diagnosed with imaging modalities such as CT or MRI.

**Clinical Pearl / High-Yield Fact**
Ultrasound is a useful tool for guiding procedures such as biopsies and injections, and it can also be used to monitor the progression of conditions such as liver disease.

**Correct Answer:** C.