Renal Physiology Practice Questions

Q: What is the functional unit of the kidney?

Nephron. ### Explanation **Correct Answer: A. Nephron** The **nephron** is the structural and functional unit of the kidney because it is the smallest unit capable of performing all the kidney's primary functions: filtration, reabsorption, secretion, and excretion. Each human kidney contains approximately **0.8 to 1.2 million nephrons**. A nephron consists of two main components: the renal corpuscle (for filtration) and the renal tubule (for selective processing of the filtrate). **Why the other options are incorrect:** * **B. Glomerulus:** This is merely a high-pressure capillary network within the renal corpuscle. While it is the site of ultrafiltration, it cannot perform reabsorption or secretion on its own. * **C. Collecting ducts:** These are tubes that receive urine from multiple nephrons. While they play a critical role in final urine concentration (via ADH), they are embryologically distinct (derived from the **ureteric bud**, whereas the nephron is derived from the **metanephric blastema**) and are generally not considered part of the individual nephron unit. * **D. Loop of Henle:** This is a specific segment of the renal tubule. It is essential for establishing the medullary osmotic gradient but is only one part of the larger nephron structure. **High-Yield Clinical Pearls for NEET-PG:** * **Embryology:** The nephron (from Bowman’s capsule to the DCT) develops from the **metanephric blastema**, while the collecting system develops from the **ureteric bud**. * **Nephron Loss:** Humans cannot regenerate new nephrons. After age 40, the number of functional nephrons decreases by about **10% every 10 years**. * **Cortical vs. Juxtamedullary Nephrons:** 85% of nephrons are cortical (short loops), while 15% are juxtamedullary (long loops), which are vital for the countercurrent multiplier system and concentrating urine.

Q: The cortical part of the collecting duct is functionally similar to which part of the kidney?

Distal convoluted tubule (DCT). The **cortical collecting duct (CCD)** and the **late distal convoluted tubule (DCT)** are often grouped together functionally as the "late distal tubule." ### **Why the Correct Answer is Right** The cortical part of the collecting duct is functionally similar to the DCT because both segments are the primary sites for **fine-tuning electrolyte and water balance** under hormonal control. 1. **Cellular Composition:** Both contain **Principal cells** (which reabsorb Na⁺ and secrete K⁺ via ENaC channels) and **Intercalated cells** (which regulate acid-base balance). 2. **Hormonal Regulation:** Both segments are sensitive to **Aldosterone** (for sodium reabsorption) and **Antidiuretic Hormone (ADH/Vasopressin)** (for water reabsorption via Aquaporin-2). In the absence of ADH, both segments remain impermeable to water. ### **Why Other Options are Wrong** * **Thin ascending limb:** This segment is purely passive, permeable to NaCl but impermeable to water. It lacks the active transport mechanisms and hormonal receptors found in the CCD. * **Thick ascending limb (TAL):** Known as the "diluting segment," its primary function is the active reabsorption of solutes via the **Na-K-2Cl symporter (NKCC2)**. It is always impermeable to water, unlike the CCD which changes permeability based on ADH. * **Medullary collecting duct:** While similar, the medullary portion is specifically characterized by its high permeability to **Urea** (facilitated by UT-A1 transporters under ADH influence), a feature not prominent in the cortical segment or DCT. ### **High-Yield Clinical Pearls for NEET-PG** * **Thiazide Diuretics** act on the early DCT (NCC transporter). * **Potassium-sparing diuretics** (e.g., Amiloride, Spironolactone) act on the late DCT and CCD. * **Liddle’s Syndrome** involves a gain-of-function mutation in the ENaC channels located in these specific segments, leading to hypertension and hypokalemia.

Q: Which substance(s) is/are freely filtered across glomerular capillaries?

Glucose. **Explanation:** The glomerular filtration barrier acts as a selective sieve based on two primary factors: **molecular size** and **electrical charge**. For a substance to be "freely filtered," its concentration in the Bowman’s space must be equal to its concentration in the plasma (Filtration Ratio = 1.0). **1. Why Glucose is Correct:** Glucose is a small, uncharged molecule with a molecular weight of approximately 180 Da. The glomerular basement membrane allows the passage of all neutral substances with a diameter of less than 4 nm (or <20,000 Da). Since glucose is well below this threshold, it is filtered completely and freely. **2. Analysis of Incorrect Options:** * **Albumin:** Although its molecular radius is small enough to potentially pass, albumin is negatively charged. The glomerular capillary wall (specifically the glycocalyx and podocytes) is lined with **heparan sulfate**, which creates a negative charge barrier that repels albumin. * **Thyroxin:** While thyroxin itself is small, over 99% of it circulates **bound to plasma proteins** (like TBG and albumin). Since large proteins cannot cross the barrier, the bound hormone remains in the blood. * **Creatinine:** *Note:* In many standard physiological contexts, creatinine is also considered freely filtered. However, in the context of this specific question format, glucose is the classic textbook example of a substance that is 100% filtered and then entirely reabsorbed in the proximal tubule. **High-Yield Clinical Pearls for NEET-PG:** * **Minimal Change Disease:** The loss of the negative charge (anionic barrier) leads to massive albuminuria, even though the structural "pores" remain intact. * **Filtration Fraction:** Normally ~20% of renal plasma flow. * **Myoglobin vs. Hemoglobin:** Myoglobin is small and freely filtered (causing renal failure in rhabdomyolysis), whereas Hemoglobin is larger and only filtered if the haptoglobin-binding capacity is exceeded.

Q: What is the major source of ammonia in the kidney?

Glutamine. **Explanation:** The kidney plays a vital role in acid-base balance by excreting hydrogen ions in the form of ammonium ($NH_4^+$). The major source of this ammonia is the amino acid **Glutamine**. **Why Glutamine is correct:** Approximately 60–80% of renal ammonia is derived from the metabolism of Glutamine, primarily in the cells of the **Proximal Convoluted Tubule (PCT)**. The process involves two key steps: 1. **Glutaminase** enzyme converts Glutamine into Glutamate, releasing one $NH_3$ molecule. 2. **Glutamate Dehydrogenase** then converts Glutamate into alpha-ketoglutarate, releasing a second $NH_3$ molecule. The resulting ammonia ($NH_3$) diffuses into the tubular lumen, where it buffers $H^+$ to form $NH_4^+$, which is then excreted. **Why other options are incorrect:** * **Glutamate:** While Glutamate is an intermediate in the production of ammonia, it is secondary to Glutamine. Glutamine is the preferred "carrier" of nitrogen from the liver and muscles to the kidneys. * **Alpha-ketoglutarate:** This is the carbon skeleton "end-product" of the deamination process. It is metabolized to produce bicarbonate ($HCO_3^-$), which is reabsorbed into the blood to further buffer systemic acidity. * **Alanine:** Alanine is the primary nitrogen carrier for the liver (Glucose-Alanine cycle) rather than the kidney. **High-Yield Clinical Pearls for NEET-PG:** * **Site of production:** Ammonia production occurs predominantly in the **PCT**. * **Regulation:** In states of **chronic metabolic acidosis**, renal ammonia production can increase significantly (up to 10-fold) to enhance $H^+$ excretion. * **Diffusion Trapping:** $NH_3$ is lipid-soluble and diffuses easily; once it combines with $H^+$ to form $NH_4^+$, it becomes water-soluble and "trapped" in the lumen for excretion.

Q: Ultrafiltrate passes through all filtration barriers except?

Fenestra in podocytes. The glomerular filtration barrier is a highly specialized trilaminar structure that filters blood to form urine. To understand why **Option C** is the correct answer, one must distinguish between the physical structures of the barrier and the terminology used to describe them. ### Explanation of the Correct Answer The glomerular filtration barrier consists of three layers: the fenestrated endothelium, the basement membrane, and the podocyte layer. While the **endothelial cells** have "fenestrae" (pores), **podocytes do not have fenestrae**. Instead, podocytes have long finger-like projections called **pedicels (foot processes)**. The spaces between these interdigitating pedicels are called **filtration slits** (bridged by a slit diaphragm). Therefore, "Fenestra in podocytes" is a non-existent anatomical feature, making it the correct choice for what the ultrafiltrate does *not* pass through. ### Analysis of Incorrect Options * **A. Endothelial fenestra:** These are large pores (70–100 nm) in the glomerular capillary wall that allow all non-cellular components of blood to pass through. * **B. Basement membrane:** This is the middle layer composed of Type IV collagen and heparan sulfate. It acts as a physical and charge-selective barrier (negative charge). * **D. Filtration slit between podocytes:** These are the final gaps (approx. 25–40 nm) through which the filtrate enters Bowman’s space. ### NEET-PG High-Yield Pearls * **Charge Barrier:** The glomerular barrier is negatively charged due to **heparan sulfate** and **sialoproteins** (like podocalyxin). This repels negatively charged proteins like albumin. * **Nephrin:** This is the key protein in the slit diaphragm. Mutations in the NPHS1 gene (encoding nephrin) lead to **Finnish-type congenital nephrotic syndrome**. * **Minimal Change Disease:** Characterized by the **effacement (flattening) of podocyte foot processes**, leading to massive proteinuria.

Q: In the proximal tubule, bicarbonate (HCO3-) reabsorption occurs via which mechanism?

Sodium-hydrogen exchanger. **Explanation:** In the proximal convoluted tubule (PCT), approximately 80–90% of filtered bicarbonate ($HCO_3^-$) is reabsorbed. This process is indirect and relies on the **Sodium-Hydrogen Exchanger (NHE3)** located on the apical membrane. **Mechanism:** 1. The NHE3 transporter pumps $H^+$ ions into the tubular lumen in exchange for $Na^+$ ions. 2. In the lumen, $H^+$ combines with filtered $HCO_3^-$ to form carbonic acid ($H_2CO_3$). 3. **Carbonic Anhydrase (Type IV)** on the brush border dissociates $H_2CO_3$ into $CO_2$ and $H_2O$. 4. $CO_2$ diffuses into the cell, where **Carbonic Anhydrase (Type II)** converts it back into $HCO_3^-$ and $H^+$. 5. The $HCO_3^-$ then exits the basolateral membrane into the blood via the $Na^+$-$HCO_3^-$ symporter (NBCe1). **Analysis of Incorrect Options:** * **Option A (NCC):** Located in the **Distal Convoluted Tubule**. It is the target of Thiazide diuretics. * **Option B (NKCC2):** Located in the **Thick Ascending Limb** of the Loop of Henle. It is the target of Loop diuretics (Furosemide). * **Option D ($Na^+$-$K^+$ Exchanger):** While the $Na^+$-$K^+$ ATPase exists on the basolateral membrane to create the gradient for $Na^+$, it is not the primary mechanism for bicarbonate reabsorption. **High-Yield Clinical Pearls for NEET-PG:** * **Acetazolamide:** Inhibits Carbonic Anhydrase, leading to bicarbonate loss in urine (Proximal Renal Tubular Acidosis/Type 2 RTA). * **Angiotensin II:** Stimulates the NHE3 exchanger, increasing $HCO_3^-$ reabsorption (explaining contraction alkalosis). * **Rate-limiting step:** The secretion of $H^+$ via the NHE3 is the primary driver for $HCO_3^-$ reclamation.

Question 1

What is the functional unit of the kidney?

Accepted Answer

Nephron

Answer

Glomerulus

Answer

Collecting ducts

Answer

Loop of Henle

Question 2

The cortical part of the collecting duct is functionally similar to which part of the kidney?

Accepted Answer

Distal convoluted tubule (DCT)

Answer

Thin ascending limb of Henle loop

Answer

Thick ascending limb of Henle loop

Answer

Medullary collecting duct

Question 3

Creatinine clearance is used to assess:

Accepted Answer

Glomerular function

Answer

Afferent loop pressure

Answer

Tubular function

Answer

None of the above

Question 4

Which substance(s) is/are freely filtered across glomerular capillaries?

Accepted Answer

Glucose

Answer

Albumin

Answer

Thyroxin

Answer

Creatinine

Question 5

What is the major source of ammonia in the kidney?

Accepted Answer

Glutamine

Answer

Glutamate

Answer

alpha-ketoglutarate

Answer

Alanine

Question 6

Ultrafiltrate passes through all filtration barriers except?

Accepted Answer

Fenestra in podocytes

Answer

Endothelial fenestra

Answer

Basement membrane

Answer

Filtration slit between podocytes

Question 7

Which statement is false regarding the ascending limb of the loop of Henle?

Accepted Answer

A defect in the Na+K+2Cl- channel leads to Pendred syndrome.

Answer

It is highly permeable to sodium.

Answer

It is permeable to magnesium.

Answer

It is a part of the countercurrent multiplier system.

Question 8

In the proximal tubule, bicarbonate (HCO3-) reabsorption occurs via which mechanism?

Accepted Answer

Sodium-hydrogen exchanger

Answer

Sodium-chloride cotransporter

Answer

Sodium-potassium-2-chloride cotransporter

Answer

Sodium-potassium exchanger

Question 9

A negatively charged molecule is filtered with more difficulty compared to a positively charged one because why?

Accepted Answer

The filtering membrane has negatively charged sialoproteins.

Answer

Negatively charged molecules are inherently larger.

Answer

The filtering membrane has positively charged proteins.

Answer

Urine is typically acidic.

Question 10

Which characteristic allows a particle to pass easily through the glomerular membrane, assuming it has the same diameter as another particle?

Accepted Answer

Positive charge

Answer

Negative charge

Answer

Neutral charge

Answer

Charge has no relation to glomerular filtration

Renal Physiology — MCQs

Renal Physiology — MCQs

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