**Core Concept**
Glycine is a non-essential, non-polar amino acid that can be synthesized by the human body through various pathways. It is a key intermediate in the metabolism of several compounds, including creatine, heme, and purines.

**Why the Correct Answer is Right**
Glycine is synthesized in the body through the breakdown of other compounds, such as serine, threonine, and choline, as well as through the metabolism of certain amino acids. It is also synthesized in the liver through the action of the enzyme serine hydroxymethyltransferase (SHMT), which catalyzes the conversion of serine to glycine and a one-carbon unit. This process is crucial for the synthesis of purines, which are essential for DNA and RNA synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glycine can be synthesized from serine through the action of SHMT.
**Option B:** This option is incorrect because glycine can be synthesized from threonine through the action of a specific dehydrogenase enzyme.
**Option C:** This option is incorrect because glycine can be synthesized from choline through the action of the enzyme choline dehydrogenase.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that glycine is an essential intermediate in the synthesis of creatine, which is vital for muscle contraction and energy production. Patients with glycine deficiency may experience muscle weakness and fatigue.

**Correct Answer:** D.