**Core Concept**
Gallstones are solid particles that form in the gallbladder, typically composed of cholesterol, bilirubin, or other substances. Complications arise when these stones obstruct bile ducts or cause inflammation.

**Why the Correct Answer is Right**
The correct answer, **C. Acute cholecystitis**, is a common complication of gallstones. When a gallstone obstructs the cystic duct, it can cause inflammation of the gallbladder, leading to acute cholecystitis. This condition is characterized by severe abdominal pain, fever, and leukocytosis. The inflammation can cause the gallbladder to become distended and tender, and may lead to gangrene or perforation if left untreated.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pancreatitis is a potential complication of gallstones, but it is more commonly associated with gallstones that obstruct the common bile duct (CBD) rather than the cystic duct. While pancreatitis can be a complication of gallstones, it is not the most direct or common complication.
* **Option B:** Choledocholithiasis refers to the presence of gallstones in the common bile duct (CBD), which can cause obstruction and jaundice. While this is a potential complication of gallstones, it is not as directly related to the formation of acute cholecystitis as option C.
* **Option D:** Carcinoma stomach (stomach cancer) and carcinoma pancreas (pancreatic cancer) are both potential complications of gallstones, but they are much less common than acute cholecystitis. These conditions are more related to chronic inflammation and malignant transformation rather than the acute obstruction caused by gallstones.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl to remember is that acute cholecystitis is a surgical emergency that requires prompt treatment to prevent complications. The American College of Surgeons recommends that patients with acute cholecystitis undergo cholecystectomy within 72 hours of diagnosis to minimize the risk of complications.

**Correct Answer:** C. Acute cholecystitis