**Core Concept**
Protein synthesis, also known as protein biosynthesis, is the process by which cells create proteins. This complex process involves the translation of messenger RNA (mRNA) into a sequence of amino acids that fold into a specific three-dimensional structure, ultimately forming a functional protein. The ribosome plays a crucial role in this process by serving as the site where translation occurs.

**Why the Correct Answer is Right**
The ribosome is a complex molecular machine composed of two subunits, the large and small subunit, which work together to read the sequence of nucleotides on the mRNA and match them to the corresponding amino acids. The ribosome uses transfer RNA (tRNA) molecules to bring the amino acids to the site of translation, where they are linked together in a specific sequence to form a polypeptide chain. This process is highly regulated, with various factors controlling the initiation, elongation, and termination of translation.

**Why Each Wrong Option is Incorrect**
**Option B:** The Golgi apparatus is involved in protein modification, sorting, and packaging, but it is not the site of protein synthesis. Instead, it receives proteins synthesized in the endoplasmic reticulum and modifies them before sending them to their final destination.
**Option C:** Lysosomes are membrane-bound organelles responsible for cellular digestion and recycling, and they do not play a direct role in protein synthesis.
**Option D:** Endosomes are involved in the sorting and processing of proteins and lipids, but they are not the primary site of protein synthesis.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that protein synthesis is a highly regulated process, and any disruption in this process can lead to various diseases, including genetic disorders and cancer. Understanding the mechanisms of protein synthesis is crucial for the development of targeted therapies.

**✓ Correct Answer: A. Ribosome. The ribosome is the site of protein synthesis.**