**Question:** Which of the following agents is used to measure glomerular filtration rate (GFR) –

A. Creatinine
B. Urea
C. Iodine-labelled substances (e.g., iohexol)
D. Indigo carmine

**Correct Answer:** C. Iodine-labelled substances (e.g., iohexol)

**Core Concept:** Glomerular filtration rate (GFR) is a measure of the rate at which blood passes through the glomeruli in the kidneys. A healthy kidney filters approximately 120-150 ml/min. Monitoring GFR is crucial in assessing kidney function, diagnosing kidney disease, and evaluating treatment efficacy.

**Why the Correct Answer is Right:** Iodine-labelled substances, such as iohexol, are used to measure GFR because they are non-toxic, non-absorbed, and easily cleared by the kidneys via glomerular filtration. When these substances are injected into the bloodstream, they travel through the nephrons and are eventually excreted in the urine. By measuring the disappearance of the contrast material from the blood over time, we can calculate the GFR.

**Why Each Wrong Option is Incorrect:**

A. Creatinine: While creatinine is a waste product that is filtered by the glomerulus, it is also reabsorbed and secreted by the kidneys. Therefore, creatinine levels in the blood do not directly represent GFR.

B. Urea: Similar to creatinine, urea levels in the blood reflect both filtration and reabsorption in the kidneys. Urea is not a suitable marker for assessing GFR.

D. Indigo carmine: Indigo carmine is a dye used for diagnostic purposes, such as enteroscopy or capsule endoscopy. It is not a suitable agent for measuring GFR.

**Clinical Pearl:** Glomerular filtration rate estimation is essential for diagnosing kidney diseases, monitoring treatment progress, and assessing renal function. Understanding the principles of GFR measurement helps healthcare professionals diagnose and manage patients with renal dysfunction effectively.