**Core Concept**
Cytidine deamination is a process that involves the removal of an amino group (-NH2) from the nucleotide cytidine, resulting in the formation of uridine. This process can occur in DNA and is an example of a type of chemical modification that can lead to gene inactivation.

**Why the Correct Answer is Right**
The process of cytidine deamination is mediated by the enzyme apurinic/apyrimidinic endonuclease (APE1) and is a key mechanism by which DNA is damaged and can lead to gene inactivation. When a cytidine residue is deaminated, it is converted to uridine, which can then be recognized and removed by the base excision repair (BER) pathway. This can result in the formation of a double-strand break in the DNA, which can lead to gene inactivation or even cell death.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the process of cytidine deamination and gene inactivation.
* **Option B:** While methylation of cytosine residues can also lead to gene inactivation, it is a different process from cytidine deamination.
* **Option C:** This option is not directly related to the process of cytidine deamination and gene inactivation.

**Clinical Pearl / High-Yield Fact**
Cytidine deamination is an example of a type of chemical modification that can lead to gene inactivation. Understanding the mechanisms by which DNA is damaged and repaired is critical for the diagnosis and treatment of genetic disorders.

**Correct Answer: B. Methylation of cytosine residues often results in gene inactivation, but this is a different process from cytidine deamination.