Which of the following is not a nucleoside ?

Options

A. Deoxydenosine
B. Cytidine
C. Riboflavin
D. Thymidine

Answer

To determine which of the following is not a nucleoside, we need to understand what a nucleoside is.

### Step-by-Step Solution:

1. **Definition of Nucleoside**:
- A nucleoside consists of a pentose sugar and a nitrogenous base. The nitrogenous bases can be classified into two categories: purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).

2. **Examine the Options**:
- The options provided are:
- Deoxyadenosine
- Cytidine
- Riboflavin
- Thymidine

3. **Identify Each Option**:
- **Deoxyadenosine**: This is a nucleoside composed of the sugar deoxyribose and the nitrogenous base adenine (a purine). Thus, it is a nucleoside.
- **Cytidine**: This is a nucleoside made up of ribose sugar and the nitrogenous base cytosine (a pyrimidine). Therefore, it is also a nucleoside.
- **Riboflavin**: This is not a nucleoside; it is actually vitamin B2. It does not fit the definition of a nucleoside as it does not consist of a sugar and a nitrogenous base.
- **Thymidine**: This is a nucleoside that consists of the sugar deoxyribose and the nitrogenous base thymine (a pyrimidine). Hence, it is a nucleoside.

4. **Conclusion**:
- Among the options, riboflavin is the only one that is not a nucleoside. Therefore, the correct answer is riboflavin.

### Final Answer:
Riboflavin is not a nucleoside.