**Core Concept**
Hemoglobin synthesis requires a specific amino acid that serves as a precursor for the production of heme, a crucial component of hemoglobin. This amino acid is a key building block in the biosynthesis of heme, which is essential for the proper functioning of red blood cells.

**Why the Correct Answer is Right**
The correct amino acid is **A. Glycine**. Glycine is the first amino acid incorporated into the heme molecule, which is then further modified to produce protoporphyrin IX. This is the first step in the biosynthesis of heme, and glycine plays a critical role in this process. The enzyme responsible for this reaction is aminolevulinic acid synthase, which catalyzes the condensation of glycine and succinyl-CoA to form aminolevulinic acid (ALA).

**Why Each Wrong Option is Incorrect**
* **Option B:** Tyrosine is an amino acid involved in various biochemical pathways, but it is not directly involved in the synthesis of heme or hemoglobin.
* **Option C:** Phenylalanine is another essential amino acid, but it is not required for hemoglobin synthesis. It can be converted to tyrosine through the action of the enzyme phenylalanine hydroxylase, but it is not directly involved in the production of heme.

**Clinical Pearl / High-Yield Fact**
Glycine is also involved in the production of creatine, a compound that plays a crucial role in muscle function and energy metabolism. This highlights the importance of glycine in various biochemical pathways and its role as a precursor for the synthesis of essential compounds.

**Correct Answer:** A. Glycine