What are the three main steps in urine formation in the correct order?

• A. Filtration, Reabsorption, Secretion
• B. Reabsorption, Filtration, Secretion
• C. Secretion, Filtration, Reabsorption
• D. Filtration, Secretion, Reabsorption

Answer: (A)

The main idea here is the sequence of how the kidneys form urine in the nephron: filtration first, then reabsorption, then secretion. Filtration happens in the glomerulus, where blood plasma is pushed into Bowman's capsule to form filtrate while large proteins and cells stay in the blood. Next, as fil trate travels through the renal tubules, the body reclaims essential substances—water, glucose, amino acids, and various ions—back into the bloodstream. Finally, secretion adds additional wastes from the blood into the tubule, mainly in the distal parts, so they’re eliminated in urine. This order makes sense because you need a filtrate to begin with, you reclaim what you want to save, and you add any remaining wastes that should be excreted. If reabsorption happened before filtration or secretion before filtrate exists, the process wouldn’t make physiological sense, since there would be nothing to reabsorb or nothing to secrete into.