**Core Concept**
Chromogenic compounds in the urine react with oxygen in the air to form dark pigments, a phenomenon known as oxidation or auto-oxidation.

**Why the Correct Answer is Right**
The correct answer is likely to be a compound that contains a chromogenic group, such as a phenolic or indolic structure. These groups can undergo oxidation reactions when exposed to oxygen, leading to the formation of dark pigments. For example, phenylalanine, a non-essential amino acid, contains a phenolic group that can undergo oxidation to form a dark pigment.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a chromogenic compound, making it an unlikely candidate.
**Option B:** This compound may not undergo oxidation reactions in the same way as chromogenic compounds.
**Option C:** This compound may not be present in urine or may not contribute to the blackening of urine.

**Clinical Pearl / High-Yield Fact**
The blackening of urine is often associated with the presence of chromogenic compounds, such as those found in certain bacterial infections or in the metabolism of certain medications.

**Correct Answer: B. Urobilin**