Which of the following defines a nucleoside?

• A. A five-carbon sugar attached to a nitrogenous base
• B. A five-carbon sugar attached to a phosphate group
• C. A nitrogenous base attached to a phosphate group
• D. A phosphate group attached to a ribose

Answer: (A)

A nucleoside is formed when a five-carbon sugar (ribose in RNA or deoxyribose in DNA) is linked to a nitrogenous base. This pairing is the basic unit that, when one or more phosphate groups are added to the sugar, becomes a nucleotide—the building block that forms the DNA and RNA backbones. The phosphate groups are what distinguish a nucleotide from a nucleoside; without the phosphate, you just have the sugar-plus-base, which is the nucleoside. So describing a five-carbon sugar attached to a nitrogenous base aligns perfectly with what defines a nucleoside, whereas the other options either omit the base, omit the sugar, or replace the base with a phosphate group.