**Core Concept**
Leiomyosarcoma is a rare, malignant tumor of the smooth muscle origin, which can occur in various locations including the gastrointestinal tract, uterus, and skin. In the stomach, leiomyosarcoma is a rare but aggressive neoplasm that can cause bleeding due to its vascular nature.

**Why the Correct Answer is Right**
Leiomyosarcoma arises from the smooth muscle cells of the stomach wall and can grow large enough to erode into nearby blood vessels, leading to bleeding. This is often the presenting symptom of leiomyosarcoma, making it the most common cause of bleeding from a stomach tumor. The tumor's rapid growth and vascular invasion are responsible for its tendency to bleed.

**Why Each Wrong Option is Incorrect**
**Option A:** Adenocarcinoma is the most common type of stomach cancer, but it typically presents with symptoms like weight loss, abdominal pain, and early satiety rather than bleeding. While adenocarcinoma can cause bleeding, it is not the most common stomach tumor to do so.

**Option B:** Squamous carcinoma is a rare type of stomach cancer that typically arises from the squamous epithelium of the stomach. It is not as commonly associated with bleeding as leiomyosarcoma.

**Option D:** Fibrosarcoma is a rare type of cancer that arises from fibrous tissue. It is not typically associated with the stomach or bleeding symptoms.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with GI bleeding, consider the possibility of a leiomyosarcoma, especially in the stomach. This rare tumor can present with sudden, severe bleeding and requires prompt diagnosis and treatment.

**✓ Correct Answer: C. Leiomyosarcoma**