**Core Concept**
Darkening of urine on standing is a clinical phenomenon related to the concentration of urine and the presence of certain compounds that become more apparent in concentrated urine. This occurs due to the reabsorption of water in the kidneys, leading to a more concentrated urine.

**Why the Correct Answer is Right**
When urine is concentrated, the concentration of certain compounds like bilirubin, urobilinogen, and myoglobin increases, causing the urine to darken. Bilirubin is a breakdown product of hemoglobin that is normally excreted in the bile, but some of it can be reabsorbed into the bloodstream and excreted in the urine. As the urine becomes more concentrated, the bilirubin becomes more apparent, causing the urine to darken. Urobilinogen is another compound that can contribute to darkening of urine, particularly in individuals with liver disease or hemolytic disorders.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not directly related to the darkening of urine on standing.
* **Option B:** While hematuria (blood in the urine) can cause darkening of urine, it is not the primary cause of darkening on standing.
* **Option C:** This option is not relevant to the darkening of urine on standing.

**Clinical Pearl / High-Yield Fact**
Remember that darkening of urine on standing can be a sign of concentrated urine, which can be a normal physiological response to dehydration or a sign of underlying kidney disease. It's essential to consider the patient's overall clinical context when evaluating this finding.

**Correct Answer:** C. Bilirubin.