Correct Option: C. Nephron Detailed Explanation: The nephron is the fundamental structural and functional unit of the kidney, and it plays a crucial role in the excretory system by filtering blood to form urine. Here’s a step-by-step breakdown of why the nephron is the correct answer:

1. Structure of the Nephron: Each kidney contains approximately one million nephrons. A nephron consists of several parts:
2. Glomerulus: A network of capillaries where blood filtration begins.
3. Bowman's Capsule: A cup-like structure that encases the glomerulus and collects the filtrate.
4. Renal Tubule: Comprising the proximal convoluted tubule, loop of Henle, and distal convoluted tubule, this part reabsorbs water, ions, and nutrients from the filtrate.
5. Collecting Duct: This structure collects urine from multiple nephrons and channels it into the renal pelvis.
6. Filtration Process: The filtration process occurs primarily in the glomerulus:
7. Blood enters the glomerulus through the afferent arteriole.
8. Due to high pressure, water, ions, and small molecules are filtered out of the blood and into the Bowman's capsule, forming a fluid called glomerular filtrate.
9. Larger molecules, such as proteins and blood cells, remain in the bloodstream because they are too large to pass through the filtration barrier.
10. Urine Formation: After filtration, the nephron continues to process the filtrate:
11. In the renal tubule, essential substances like glucose and certain ions are reabsorbed back into the bloodstream.
12. Water reabsorption occurs in various parts of the nephron, particularly in the loop of Henle and collecting duct, which helps concentrate the urine.
13. The remaining waste products and excess substances are excreted as urine.

Why Other Options Are Incorrect:

• A. Ureter: The ureter is a tube that carries urine from the kidneys to the bladder. It does not filter blood; rather, it transports urine that has already been formed in the kidneys.
• B. Bladder: The bladder is a storage organ for urine. It holds urine until it is excreted from the body. It does not participate in the filtration process.
• D. Urethra: The urethra is the duct through which urine is expelled from the bladder to the outside of the body. Like the bladder, it does not filter blood; it is involved in the excretion of urine.

Summary of Key Points:

• The nephron is the primary unit responsible for filtering blood and forming urine in the kidneys.
• The filtration process occurs in the glomerulus, where blood is filtered to create glomerular filtrate.
• The renal tubule reabsorbs essential substances and water, concentrating the urine.
• Other structures like the ureter, bladder, and urethra are involved in urine transport and storage, not filtration.

Revision Summary:

• Nephron: The functional unit of the kidney responsible for blood filtration and urine formation.
• Filtration: Occurs in the glomerulus, where blood is filtered into Bowman's capsule.
• Urine Processing: Involves reabsorption of water and nutrients in the renal tubule.
• Other Structures: Ureter, bladder, and urethra are involved in urine transport and storage, not filtration.