**Core Concept**
Endoscopic ultrasound (EUS) is a diagnostic modality that combines endoscopy and ultrasound to produce high-quality images of the gastrointestinal tract and surrounding tissues. It is particularly useful in staging gastrointestinal tumors by assessing the depth of invasion and lymph node involvement.

**Why the Correct Answer is Right**
EUS is extremely useful in staging **esophageal**, **gastric**, and **pancreatic** tumors, but it is most notably beneficial for **esophageal** and **rectal** cancers, as well as for assessing **lymph nodes**. The proximity of the endoscope to the tumor allows for precise visualization of the tumor's depth and spread to adjacent structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while EUS can be used for various tumors, the question seeks the most specific and extremely useful application.
**Option B:** Incorrect as it is not the most specific answer, although EUS can be used in this context.
**Option C:** Incorrect because, although useful, it's not the answer the question is looking for based on common EUS applications.
**Option D:** Incorrect for similar reasons as options A and C.

**Clinical Pearl / High-Yield Fact**
EUS is a critical tool for staging gastrointestinal malignancies, especially for assessing the depth of tumor invasion (T-staging) and evaluating regional lymph nodes (N-staging), which are crucial for determining the appropriate treatment plan.

**Correct Answer:** D. Esophageal tumor