## **Core Concept**
Mucins are high-molecular-weight glycoproteins produced by epithelial cells. They are a key component of mucus, which protects and lubricates the surfaces of the respiratory, gastrointestinal, and genitourinary tracts. The question tests the knowledge of which glands or cells are responsible for mucin secretion.

## **Why the Correct Answer is Right**
The correct answer, **D. Pancreatic acinar cells**, is right because pancreatic acinar cells primarily secrete digestive enzymes like amylase, lipase, and trypsinogen into the pancreatic juice. Unlike Brunner's glands in the duodenum or the intestinal goblet cells, pancreatic acinar cells do not secrete mucin.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Brunner's glands are located in the duodenum and secrete mucin, which helps protect the duodenal lining from acidic chyme.
- **Option B:** Goblet cells, found throughout the respiratory and gastrointestinal tracts, are specialized for mucin secretion.
- **Option C:** Intestinal glands (or crypts of Lieberkühn) also contain cells that can secrete mucin, contributing to the protective mucus layer.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while pancreatic juice does contain some mucins, the primary source of mucin secretion in the gastrointestinal tract is from goblet cells and Brunner's glands. Pancreatic juice's main function is enzymatic digestion.

## **Correct Answer:** D. Pancreatic acinar cells.