## Core Concept
The biosynthesis of purine ribonucleotides involves the assembly of the purine ring from several amino acids, which provide the necessary carbon and nitrogen atoms. The key amino acids involved in this process are glycine, glutamine, and aspartate.

## Why the Correct Answer is Right
The purine ring is assembled on a ribose-5-phosphate backbone, and the process involves several steps, each catalyzed by a specific enzyme. Glycine contributes carbon atoms 4 and 5, and a nitrogen atom. Glutamine provides nitrogen atoms at positions 1 and 9 of the purine ring. Aspartate contributes a nitrogen atom at position 7.

## Why Each Wrong Option is Incorrect
- **Option A:** Glycine is directly involved in purine biosynthesis, providing carbon atoms 4 and 5 and a nitrogen atom.
- **Option B:** Glutamine is crucial, providing nitrogen atoms at positions 1 and 9 of the purine ring.
- **Option C:** Aspartate contributes a nitrogen atom at position 7 of the purine ring.

## Why Option D is Correct
- **Option D:** Tyrosine is not directly involved in the biosynthesis of purine ribonucleotides. While tyrosine is an important amino acid with various roles in metabolism and signaling, it does not contribute to purine ring assembly.

## Clinical Pearl / High-Yield Fact
A key point to remember is that purine synthesis is an energy-requiring process, and defects in the enzymes involved can lead to disorders like gout. The de novo purine synthesis pathway is critical for cells that require a constant supply of purines, such as rapidly dividing cells.

## Correct Answer: D. Tyrosine