**Core Concept**
Covalent modification refers to a post-translational modification of proteins, where a functional group is added or removed from the protein, altering its activity, localization, or stability. This process involves the formation of a covalent bond between the protein and the modifying group.

**Why the Correct Answer is Right**
Covalent modification involves the direct addition or removal of a functional group to a protein, resulting in a change to its structure or function. This process is often mediated by enzymes, such as kinases and phosphatases, which transfer a phosphate group to or from a specific site on the protein. The addition or removal of a phosphate group can activate or inactivate the protein, respectively. For example, the enzyme protein kinase A (PKA) adds a phosphate group to specific serine or threonine residues on target proteins, leading to their activation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because allosteric modulation involves the binding of a ligand to a site on a protein, causing a conformational change that affects the protein's activity.
**Option B:** This option is incorrect because receptor-mediated endocytosis involves the internalization of a ligand-receptor complex, which can lead to changes in cellular signaling, but is not a covalent modification.
**Option C:** This option is incorrect because protein degradation involves the breakdown of a protein into smaller peptides or amino acids, which is not a covalent modification.

**Clinical Pearl / High-Yield Fact**
Covalent modification is a critical mechanism for regulating protein activity in many cellular processes, including signal transduction, metabolism, and gene expression. Understanding covalent modification is essential for appreciating the complex interactions between proteins and their roles in disease.

**Correct Answer: C. Protein degradation is not a covalent modification.