**Question:** A nucleoside consists of:

A. A sugar molecule
B. A nitrogenous base
C. A sugar molecule and a phosphate group
D. A nitrogenous base and a sugar molecule

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

**Core Concept:**
Nucleosides are the building blocks of nucleic acids, which play a crucial role in storing and transmitting genetic information. Nucleic acids include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleosides consist of a nitrogenous base and a sugar molecule called deoxyribose or ribose, depending on whether it is DNA or RNA.

**Why the Correct Answer is Right:**
Nucleosides contain a nitrogenous base and a sugar molecule because:
1. The nitrogenous base carries the genetic information, representing the four bases: adenine (A), guanine (G), cytosine (C), and thymine (T) for DNA or adenine (A), guanine (G), cytosine (C), and uracil (U) for RNA.
2. The sugar molecule, deoxyribose or ribose, provides the structural framework for nucleic acids, making them double-stranded or single-stranded, respectively.

**Why Each Wrong Option is Incorrect:**
A) A nucleoside does not have a phosphate group, which is present in nucleotides (consisting of one sugar molecule, one nitrogenous base, and a phosphate group).
B) A nitrogenous base alone is not considered a nucleoside; it is the nitrogenous base that carries genetic information, not nucleosides.
C) A nucleoside only has a sugar molecule and a phosphate group, lacking the essential nitrogenous base.

**Clinical Pearl:**
Understanding the composition of nucleosides helps in understanding the structure and function of nucleic acids, as well as various molecular processes like DNA replication, transcription, and translation involved in gene expression and inheritance. Knowledge of nucleosides also plays a role in the diagnosis and treatment of genetic disorders, such as Tay-Sachs disease and Cystic Fibrosis, which are linked to mutations in the DNA or RNA synthesis process.