**Question:** Major site of protein glycosylation is

A. Golgi apparatus
B. Mitochondria
C. Endoplasmic reticulum
D. Nucleus

**Correct Answer:** A. Golgi apparatus

**Core Concept:**
Protein glycosylation is a post-translational modification process that occurs in cells. It involves the attachment of carbohydrates (sugars) to proteins, which plays a crucial role in protein folding, stability, and function. This process takes place in specific organelles within the cell.

**Why the Correct Answer is Right:**
The correct answer, the Golgi apparatus, is the site where protein glycosylation occurs after it has been synthesized in the endoplasmic reticulum (ER). The Golgi apparatus further modifies the proteins by adding, removing, or altering sugar moieties, resulting in the maturation of the proteins before they are transported to their final destinations within the cell or secreted into the extracellular space.

**Why Each Wrong Option is Incorrect:**

A. Mitochondria: Mitochondria are responsible for cellular respiration and energy production and do not involve in protein glycosylation.
B. Endoplasmic Reticulum (ER): The ER is where proteins are initially synthesized, but it is not the major site of glycosylation. The actual glycosylation occurs in the Golgi apparatus.
C. Nucleus: The nucleus controls gene expression and does not directly participate in protein glycosylation.
D. Nucleus: Similar to option C, the nucleus regulates gene expression and is not directly involved in protein glycosylation.

**Clinical Pearl:**
Understanding the cellular processes like protein glycosylation is essential for medical professionals as it contributes to an in-depth understanding of cellular functions, protein synthesis, and post-translational modifications. This knowledge can be applied in various fields, including diagnosis, treatment, and drug development.