**Core Concept**
The tubular maximum (Tm) is a measure of the maximum rate at which a substance can be reabsorbed or secreted by the renal tubules. It is an important concept in nephrology, as it helps to understand the renal handling of various substances, including glucose, amino acids, and ions.

**Why the Correct Answer is Right**
The correct answer is Glucose. Glucose is actively reabsorbed by the renal tubules through a sodium-glucose cotransporter (SGLT2) in the proximal convoluted tubule (PCT). The Tm for glucose is approximately 375 mg/min, which is relatively high compared to other substances. This is because glucose is an essential nutrient that needs to be reabsorbed by the kidneys to maintain proper blood glucose levels.

* Option B: Amino acids have a lower Tm compared to glucose, approximately 200-300 mg/min. This is because amino acids are not as essential for the body as glucose, and their reabsorption is not as critical.
* Option C: Sodium has a very high Tm, approximately 20,000-40,000 mmol/min. This is because sodium is a crucial ion that needs to be reabsorbed by the kidneys to maintain proper fluid balance and blood pressure.
* Option D: Urea has a very low Tm, approximately 40-50 mmol/min. This is because urea is a waste product that is primarily excreted by the kidneys, and its reabsorption is not as critical.

**Clinical Pearl / High-Yield Fact**
The Tm for glucose is an important concept in the diagnosis of diabetes mellitus. In diabetes, the kidneys are unable to reabsorb glucose effectively, leading to glucose spillage into the urine, also known as glycosuria.

**Correct Answer:** C. Sodium has a very high tubular transport maximum compared to the other options.