Receptor-mediated endocytosis is by
**Core Concept**
Receptor-mediated endocytosis is a process by which cells internalize specific molecules, such as proteins or lipids, through the formation of vesicles that engulf these molecules. This process is mediated by specific receptors on the cell surface that bind to the target molecule, leading to the invagination of the plasma membrane and the formation of an endocytic vesicle.
**Why the Correct Answer is Right**
The correct answer involves the process of clathrin-mediated endocytosis, which is a type of receptor-mediated endocytosis. In this process, clathrin, a protein that forms a lattice structure, coats the plasma membrane and helps to invaginate it, forming a vesicle that contains the receptor-ligand complex. This process is essential for the internalization of various molecules, including growth factors, hormones, and nutrients.
**Why Each Wrong Option is Incorrect**
* **Option A:** Macropinocytosis is a type of endocytosis that involves the formation of large vesicles that engulf a large amount of fluid and dissolved molecules, but it is not receptor-mediated.
* **Option B:** Phagocytosis is a type of endocytosis that involves the engulfment of large particles or cells by cells, but it is not receptor-mediated.
* **Option C:** Cytoskeleton-mediated endocytosis involves the use of the cytoskeleton to form vesicles, but it is not a specific type of receptor-mediated endocytosis.
**Clinical Pearl / High-Yield Fact**
Receptor-mediated endocytosis is essential for the internalization of various molecules, including growth factors and hormones, which play critical roles in cellular signaling and regulation.
**Correct Answer: C. Clathrin-mediated endocytosis**