**Core Concept**
Sub-mucosal glands are a type of exocrine gland located beneath the mucosa of various organs, including the respiratory, gastrointestinal, and genitourinary tracts. These glands play a crucial role in producing and secreting mucins, enzymes, and other substances that aid in lubrication, digestion, and protection of the mucosal surface.

**Why the Correct Answer is Right**
Sub-mucosal glands are responsible for producing a significant amount of mucus that helps to protect and lubricate the mucosal surface. In the respiratory tract, sub-mucosal glands, particularly the submucosal glands of the bronchi, secrete a type of mucus that helps to trap dust particles and other foreign substances, preventing them from entering the lungs. This is achieved through the action of mucin, a glycoprotein produced by the glandular epithelial cells of the sub-mucosal glands.

**Why Each Wrong Option is Incorrect**
**Option A:** While sub-mucosal glands are indeed found in the gastrointestinal tract, they are not the primary type of gland present in this region.

**Option B:** Sub-mucosal glands are not typically associated with the skin, which instead contains sebaceous and sweat glands.

**Option C:** The pancreas contains both endocrine and exocrine glands, but the sub-mucosal glands are not the primary type present in this organ.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sub-mucosal glands play a vital role in maintaining the mucosal barrier and preventing the entry of foreign substances into the body. This is particularly important in the respiratory tract, where the sub-mucosal glands help to prevent infections and diseases such as bronchitis and pneumonia.

**Correct Answer:** B.