## Core Concept
Guanido acetic acid is an intermediate in the biosynthesis of **creatine**, a crucial molecule for energy storage and transfer in muscle and brain tissue. The synthesis of guanido acetic acid involves the combination of **arginine** and **glycine**.

## Why the Correct Answer is Right
Guanido acetic acid is formed in the **kidney** from **arginine** and **glycine** through a two-step reaction. The first step involves the formation of guanido acetic acid by the enzyme **L-arginine:glycine amidinotransferase** (AGAT), which catalyzes the transfer of an amidine group from arginine to glycine. This reaction occurs primarily in the kidneys.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although the liver is involved in the next step of creatine synthesis (converting guanido acetic acid to creatine), it is not the primary site for the formation of guanido acetic acid.
- **Option B:** This option might seem plausible due to the role of the liver in metabolism, but it is not directly involved in the initial formation of guanido acetic acid.
- **Option D:** This option is incorrect because the brain is not primarily involved in the synthesis of guanido acetic acid.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **creatine supplementation** is often used by athletes to enhance performance, but its synthesis starts with guanido acetic acid formation in the kidneys. A deficiency in AGAT or other enzymes in this pathway can lead to **creatine deficiency syndromes**, highlighting the importance of understanding this biosynthetic pathway.

## Correct Answer: C. kidney, arginine and glycine.