**Core Concept**
Creatine is a crucial energy-boosting molecule for muscle contractions. Its biosynthesis involves the conversion of arginine, glycine, and methionine into creatine. The immediate precursor of creatine plays a pivotal role in this process.

**Why the Correct Answer is Right**
The immediate precursor of creatine is guanidinoacetate, which is synthesized from arginine and glycine. The enzyme guanidinoacetate N-methyltransferase (GAMT) catalyzes the methylation of guanidinoacetate to produce creatine. This reaction occurs mainly in the kidneys and liver. Guanidinoacetate is the critical intermediate that eventually gets converted into creatine.

**Why Each Wrong Option is Incorrect**
**Option A:** Arginine - While arginine is an essential amino acid involved in the biosynthesis of creatine, it is not the immediate precursor of creatine.
**Option B:** Methionine - Methionine is another amino acid that contributes to creatine biosynthesis but is not the immediate precursor.
**Option C:** Glycine - Glycine is an amino acid involved in the formation of guanidinoacetate but is not the immediate precursor of creatine itself.

**Clinical Pearl / High-Yield Fact**
A deficiency in guanidinoacetate N-methyltransferase (GAMT) enzyme can lead to creatine deficiency syndrome, characterized by muscle weakness, seizures, and developmental delays.

**Correct Answer: C. Glycine**