**Core Concept**
Muscle breakdown and protein catabolism can be estimated by measuring the levels of 3-methylhistidine in urine. This is because 3-methylhistidine is a non-essential amino acid that is produced during the breakdown of muscle protein, specifically actin and myosin.

**Why the Correct Answer is Right**
3-methylhistidine is not reabsorbed by the kidneys and is excreted in the urine. Therefore, measuring the levels of 3-methylhistidine in urine can provide an indirect estimate of muscle protein breakdown. This is particularly useful in clinical settings where muscle wasting or catabolic states are suspected, such as in patients with cancer, burns, or chronic illness.

**Why Each Wrong Option is Incorrect**
* **Option A:** Creatinine is a product of muscle breakdown, but it is not specific to muscle protein catabolism and can be influenced by other factors such as kidney function.
* **Option B:** Urea is a product of protein catabolism, but it is not specific to muscle protein breakdown and can be influenced by other factors such as dietary protein intake.
* **Option C:** Glucose is a product of carbohydrate metabolism, not protein catabolism, and is not related to muscle protein breakdown.

**Clinical Pearl / High-Yield Fact**
Remember that 3-methylhistidine is a specific marker of muscle protein breakdown, and its estimation in urine can provide valuable information in clinical settings where muscle wasting or catabolic states are suspected.

**Correct Answer: C. Glucose**