Which compound forms a red-colored complex with Ehrlich's reagent?

Urobilinogen is a compound that forms a red-colored complex when it reacts with Ehrlich's reagent. This reagent is a solution of p-dimethylaminobenzaldehyde, which, in the presence of urobilinogen, undergoes a chemical reaction to produce a colored compound. This color change is significant and is often used in clinical laboratories to detect and quantify urobilinogen levels in urine, providing important insights into liver function and hemolysis.

In contrast, the other substances listed do not produce a red complex with Ehrlich's reagent. Creatinine is a waste product from muscle metabolism and does not react in the same manner. Hemoglobin is primarily involved in oxygen transport and does not form a red complex under these conditions. Cholesterol is a lipid and does not participate in this specific chemical reaction either. Therefore, the ability of urobilinogen to form a distinguishable red-colored complex with Ehrlich's reagent is a key characteristic that aids in laboratory diagnostics.