Which of the following is not a nucleoside ?

Options

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

Answer

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

### Step 1: Define Nucleoside
A nucleoside is a compound that consists of a nucleobase (which can be a purine or pyrimidine) attached to a sugar molecule (either ribose or deoxyribose). Nucleosides are formed from the hydrolysis of nucleic acids.

### Step 2: Analyze the Options
Now let's analyze the options provided:

- **Option A: Deoxyadenosine**
- This is a nucleoside formed from the nucleobase adenine and deoxyribose sugar. It is indeed a nucleoside.

- **Option B: Cytidine**
- This is a nucleoside formed from the nucleobase cytosine and ribose sugar. It is also a nucleoside.

- **Option C: Riboflavin**
- Riboflavin is not a nucleoside; it is a vitamin (Vitamin B2) that plays a crucial role in energy production and metabolism. It does not consist of a nucleobase and a sugar.

- **Option D: Thymidine**
- This is a nucleoside formed from the nucleobase thymine and deoxyribose sugar. It is a nucleoside as well.

### Step 3: Conclusion
From the analysis, we can conclude that the option that is not a nucleoside is **Option C: Riboflavin**.

### Final Answer
**C. Riboflavin**