**Core Concept**
The Maltese cross is a characteristic feature of a specific type of gallstone. It is a radiographic sign that refers to the appearance of a gallstone on a plain abdominal radiograph. The Maltese cross is formed by the overlapping of calcium carbonate crystals within the gallstone, giving it a distinctive pattern.

**Why the Correct Answer is Right**
The Maltese cross is characteristic of gallstones that contain calcium carbonate, which is a type of cholesterol gallstone. These gallstones form when there is an imbalance in the composition of bile, leading to the precipitation of calcium carbonate crystals. The Maltese cross sign is a classic radiographic finding in patients with this type of gallstone.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it does not describe the characteristic feature of the Maltese cross.
**Option B:** is incorrect because it does not relate to the radiographic sign of the Maltese cross.
**Option C:** is incorrect because it is not associated with the specific type of gallstone that forms a Maltese cross.

**Clinical Pearl / High-Yield Fact**
The Maltese cross is a classic radiographic sign that should prompt the clinician to consider gallstones as a potential cause of abdominal pain. It is essential to remember that the Maltese cross is a specific type of gallstone that requires prompt medical attention.

**Correct Answer: D. Gallstones.**