The reagent p-dimethylaminobenzaldehyde is used to measure which of the following?

• A. Urobilinogen
• B. Total bilirubin
• C. Ammonia
• D. Alkaline phosphatase

Answer: (A)

The key idea is that p-dimethylaminobenzaldehyde acts in the Ehrlich reaction to detect urobilinogen. In acidic solution, urobilinogen reacts with this aldehyde to form a colored adduct, typically a pink to red chromogen, whose intensity can be measured spectrophotometrically. This reaction is specific for urobilinogen among the listed substances because bilirubin, ammonia, and alkaline phosphatase are detected by entirely different chemistries (bilirubin by the diazo reaction, ammonia by Nessler’s or enzymatic methods, and alkaline phosphatase by hydrolysis of a phosphate substrate). Therefore, this reagent is used to measure urobilinogen.