**Core Concept**
Iron deficiency anemia, characterized by a low Mean Corpuscular Volume (MCV), often results from chronic blood loss, which can be occult (hidden). The negative fecal occult blood test suggests that the blood loss is not significant enough to be detected through this test.

**Why the Correct Answer is Right**
A low MCV indicates microcytic anemia, which is often associated with iron deficiency. In the context of gastrointestinal disease, chronic blood loss from the upper gastrointestinal tract, such as from ulcers or gastritis, can lead to iron deficiency anemia. The negative fecal occult blood test suggests that the blood loss is likely from the upper GI tract, rather than the lower GI tract.

**Why Each Wrong Option is Incorrect**
**Option A:** Crohn's disease or ulcerative colitis, which are forms of inflammatory bowel disease (IBD), typically present with macrocytic anemia (high MCV) due to malabsorption of vitamin B12 and folate.
**Option B:** Celiac disease can cause malabsorption of iron, but it is more commonly associated with macrocytic anemia due to folate and vitamin B12 deficiency.
**Option C:** Irritable bowel syndrome (IBS) is a functional disorder that does not typically cause anemia.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with iron deficiency anemia, it is essential to consider the possibility of occult blood loss from the upper GI tract, even if the fecal occult blood test is negative.

**Correct Answer:** C.