**Core Concept**
A nucleoside is a compound formed by the combination of a nitrogenous base and a sugar molecule, typically ribose or deoxyribose, but lacking a phosphate group. This structure is crucial for the synthesis and stability of nucleic acids, such as DNA and RNA.

**Why the Correct Answer is Right**
The correct answer is a combination of a nitrogenous base and a sugar molecule. This is because nucleosides are fundamental building blocks of nucleic acids, serving as the precursors to nucleotides. The nitrogenous bases (adenine, guanine, cytosine, thymine, and uracil) pair with each other through hydrogen bonding, while the sugar molecule provides the backbone for these molecules. The absence of a phosphate group distinguishes nucleosides from nucleotides.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it lacks a clear description of the components that make up a nucleoside.
* **Option B:** This option is incorrect because it incorrectly includes a phosphate group, which is characteristic of nucleotides, not nucleosides.
* **Option C:** This option is incorrect because it does not specify the type of sugar molecule present in a nucleoside.

**Clinical Pearl / High-Yield Fact**
Remember that nucleosides are the precursors to nucleotides, which are the actual building blocks of DNA and RNA. Understanding the difference between these two is crucial for grasping the synthesis and stability of nucleic acids.

**Correct Answer: A. A nitrogenous base and a sugar molecule.**