## **Core Concept**
The process of sorting out protein synthesis refers to the cellular mechanism that directs proteins to their correct destinations within or outside the cell. This is crucial for the proper functioning of cells, especially in eukaryotic cells where proteins can be synthesized in the cytoplasm but need to be transported to various organelles or secreted. The **endoplasmic reticulum (ER)**, particularly the **rough ER**, plays a significant role in this process.

## **Why the Correct Answer is Right**
The correct answer, **C. Endoplasmic Reticulum (ER)**, is right because the ER, especially the rough ER with its attached ribosomes, is the site where proteins destined for secretion or for insertion into cellular membranes are initially synthesized and sorted. These proteins are synthesized into the ER lumen and can then be transported to the **Golgi apparatus** for further modification and sorting to other parts of the cell or for secretion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although **mitochondria** do have their own ribosomes for synthesizing some of their proteins, they are not the primary site for sorting out protein synthesis for the entire cell.
- **Option B:** This option is incorrect as **Peroxisomes** are involved in the breakdown of fatty acids and amino acids, and while they do contain proteins, they are not primarily involved in the sorting of protein synthesis.
- **Option D:** This option is incorrect because **Lysosomes** are involved in digestion and recycling of cellular waste and do not play a primary role in the initial sorting of protein synthesis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that proteins destined for the **secretory pathway** (e.g., insulin, digestive enzymes) are initially synthesized into the ER. This process involves **signal sequences** on the proteins that target them to the ER for further processing and sorting.

## **Correct Answer: C. Endoplasmic Reticulum (ER).**