Definition check: A nucleoside is best described as a purine or pyrimidine base that is __________ to a pentose sugar.

Introduction / Context:
Precise terminology—nucleoside versus nucleotide—is foundational in biochemistry. Mislabeling can cause confusion in pathways like salvage, phosphorylation, and nucleic acid synthesis.

Given Data / Assumptions:

• Nucleoside = base + sugar.
• Nucleotide = nucleoside + phosphate group(s).
• Bond type between base and sugar is asked.

Concept / Approach:
A nucleoside has a nitrogenous base linked to a pentose sugar via a covalent N-glycosidic bond (N9 of purines or N1 of pyrimidines to C1' of the sugar). Ionic or hydrogen bonds do not define the core nucleoside linkage.

Step-by-Step Solution:
Identify components: base + sugar.Recall linkage: N-glycosidic covalent bond.Therefore, “covalently bonded” is correct.

Verification / Alternative check:
Hydrolysis under acidic conditions cleaves the glycosidic bond—consistent with a covalent linkage.

Why Other Options Are Wrong:

• Ionic and hydrogen bonds are noncovalent and do not define nucleosides.
• “None of the above” is incorrect because the covalent option is valid.

Common Pitfalls:
Confusing nucleosides with nucleotides (which include phosphate).

Final Answer:
covalently bonded