**Core Concept**
Pseudocyst in pancreatitis is a fluid collection that forms outside the pancreas, typically in the setting of acute or chronic pancreatitis. These cysts are usually filled with pancreatic juice and can cause various symptoms due to their location and size.

**Why the Correct Answer is Right**
The usual presenting symptom of a pseudocyst in pancreatitis is abdominal pain, often located in the upper or middle abdomen. This pain can be dull and persistent, and may radiate to the back. The pain is usually caused by the cyst's size and location, which can put pressure on surrounding structures, including the stomach, small intestine, and spleen. As the cyst grows, it can also cause nausea, vomiting, and weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Nausea and vomiting are common symptoms of pancreatitis, but they are not the usual presenting symptom of a pseudocyst.
**Option B:** Weight loss is a possible consequence of a pseudocyst, but it is not the primary presenting symptom.
**Option C:** Diarrhea can occur in pancreatitis, but it is not typically associated with pseudocysts.

**Clinical Pearl / High-Yield Fact**
A pseudocyst in pancreatitis is usually diagnosed on imaging studies, such as CT or ultrasound, and may require drainage if it is large or causing symptoms. It's essential to distinguish pseudocysts from true cysts, which have a clear connection to the pancreatic duct.

**Correct Answer: C. Abdominal pain.**