**Question:** A 12 year old boy is brought to emergency by his parents with complaints of severe polydipsia and polyuria. Laboratory examination reveals a purple ring when a test was done in his urine. Which of the following is the most likely source of this compound which is positive in this patient?

A. Urea
B. Uric Acid
C. Creatinine
D. Hemoglobin

**Core Concept:**

The purple ring in the urine of the 12-year-old boy is indicative of a positive test for a nitrogenous compound. This compound is typically associated with conditions causing hemolysis or increased production of free hemoglobin. In this case, the purple ring is the result of a positive dipstick test for hemoglobin (Hgb) or its breakdown products.

**Why the Correct Answer is Right:**

The correct answer is D, Hemoglobin (Hgb). This is because the purple ring in the urine is indicative of the presence of a nitrogenous compound, specifically the breakdown products of hemoglobin. Hemolysis (breakdown of red blood cells) or increased production of free hemoglobin can lead to the presence of these compounds in the urine. Hemoglobinuria is a common presentation in conditions like hemolysis (e.g., due to drugs, infections, or trauma), hemoglobinopathies (e.g., sickle cell anemia), or hemoglobinuria (e.g., from transfusion reactions).

**Why Each Wrong Option is Incorrect:**

A) Urea: Urea is a nitrogenous compound, but it is not responsible for the purple ring in urine. Urea is the end product of protein catabolism in the liver and is not typically associated with hemolysis or hemoglobinuria.

B) Uric Acid: Uric acid is a product of purine catabolism and is not directly related to hemolysis or hemoglobinuria. Although high levels of uric acid can indirectly indicate hemolysis due to increased production of bilirubin, it is not the direct cause of the purple urine ring.

C) Creatinine: Creatinine is a waste product of muscle protein catabolism and is not associated with hemolysis or hemoglobinuria. Creatinine levels are influenced by muscle mass but do not indicate hemolysis or hemoglobinuria.

**Clinical Pearl:**

Hemoglobinuria is a rare but important clinical finding, as it can point to underlying conditions such as hemolysis, hemoglobinopathies, or transfusion reactions. In this scenario, the purple urine ring indicates hemoglobinuria due to hemolysis or hemoglobinopathies in the kidneys. Further workup is essential to determine the underlying cause, including blood tests for hemoglobin, bilirubin, and lactate dehydrogenase (LDH) levels, as well as haematology investigations to assess for hemolysis and hemoglobinopathies.