**Core Concept**
The question is testing the student's knowledge of nucleotide biosynthesis, specifically the sources of nitrogen atoms in purine and pyrimidine rings. This involves understanding the biochemical pathways of nucleotide synthesis.

**Why the Correct Answer is Right**
The correct answer involves identifying the common nitrogen source for both purine and pyrimidine synthesis. In purine synthesis, nitrogen atoms come from aspartate, glutamine, and glycine, among others. For pyrimidines, the nitrogen atoms are derived from aspartate and glutamine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while aspartate does contribute a nitrogen atom, it is not the sole or most direct common source for both.
**Option B:** Incorrect because, although glutamine is a nitrogen source, the question seems to seek a more direct common donor.
**Option C:** Incorrect as it doesn't directly pertain to the common nitrogen source for both purine and pyrimidine synthesis.
**Option D:** This is the correct source, as both purine and pyrimidine rings obtain nitrogen from this source, but the explanation for why the correct answer is right was started, implying the correct choice is related to a common nitrogen source like glutamine or aspartate.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that both purine and pyrimidine synthesis require nitrogen from amino acids, highlighting the importance of nitrogen metabolism in nucleotide synthesis. Glutamine is particularly notable as it donates amino groups in both pathways.

**Correct Answer:** D. Glutamine