**Core Concept**
Schilling's test is a radiolabeled in vivo test used to assess the absorption of vitamin B12, specifically to diagnose pernicious anemia. It involves oral administration of radioactive vitamin B12 and subsequent measurement of its excretion in the urine. The test relies on the presence of intrinsic factor, a protein produced by the stomach's parietal cells, to facilitate vitamin B12 absorption.

**Why the Correct Answer is Right**
The test is false-positive in conditions where the body is able to absorb vitamin B12 in the absence of intrinsic factor, such as in cases of ileal resection or bacterial overgrowth. In these conditions, the ileal mucosa is able to absorb vitamin B12 through mechanisms other than intrinsic factor-mediated absorption. This can lead to a false-positive result, as the test may indicate adequate vitamin B12 absorption when, in fact, it is not being absorbed through the normal pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because autoimmune gastritis primarily affects the parietal cells, leading to a deficiency in intrinsic factor and a true-positive result on Schilling's test.
**Option B:** This option is incorrect because Crohn's disease is a type of inflammatory bowel disease that can lead to malabsorption of vitamin B12, but it does not typically cause a false-positive result on Schilling's test.
**Option C:** This option is incorrect because celiac disease is an autoimmune reaction to gluten that primarily affects the small intestine's villi, leading to malabsorption of nutrients, including vitamin B12.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Schilling's test is rarely used today due to the availability of more accurate and convenient tests for vitamin B12 deficiency, such as measurement of serum vitamin B12 and methylmalonic acid levels.

**Correct Answer:** C.