A substance present in the glomerular filtrate in the mammalian Kidney is

Analyze the question: It asks which substance is found in the glomerular filtrate, the fluid forced out of the blood in the glomerulus into the Bowman's capsule.

Explain the concept: Ultrafiltration happens in the glomerulus. High hydrostatic pressure forces small molecules out of the blood capillaries through the capillary wall, the basement membrane and the podocytes of the Bowman's capsule. These layers act like a sieve:

• Small molecules pass through: water, glucose, amino acids, urea, mineral salts (e.g. Na⁺, Cl⁻), and vitamins.
• Large molecules and cells stay in the blood: blood cells (red cells, white cells), platelets, and large plasma proteins such as albumen (albumin), fibrinogen and globulins.

Step-by-step solution:

• A. Glucose: a small molecule, so it is filtered into the Bowman's capsule. It is later fully reabsorbed in the proximal convoluted tubule. ✔
• B. Fibrinogen: a large plasma protein, so it is too big to be filtered. ✘
• C. Albumen: a large plasma protein, so it is also retained in the blood. ✘
• D. Blood platelets: cell fragments, much too large to pass through. ✘

Answer: Glucose is the only option small enough to be in the glomerular filtrate, so the answer is A.

Exam tip: Remember "small passes, big stays." If protein or blood cells show up in a person's urine, it suggests damage to the filtration barrier, as in kidney disease. Glucose in the filtrate is normal, but glucose in the urine is not, since it should all be reabsorbed (and appears in the urine in diabetes).