Renal Physiology — MCQs

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 311: What is the functional unit of the kidney?

A. Nephron (Correct Answer)
B. Glomerulus
C. Collecting ducts
D. Loop of Henle

Explanation: ### 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.

Question 312: The cortical part of the collecting duct is functionally similar to which part of the kidney?

A. Distal convoluted tubule (DCT) (Correct Answer)
B. Thin ascending limb of Henle loop
C. Thick ascending limb of Henle loop
D. Medullary collecting duct

Explanation: 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.

Question 313: Creatinine clearance is used to assess:

A. Glomerular function (Correct Answer)
B. Afferent loop pressure
C. Tubular function
D. None of the above

Explanation: **Explanation:** **1. Why Glomerular Function is Correct:** Creatinine clearance ($C_{Cr}$) is the most common clinical method used to estimate the **Glomerular Filtration Rate (GFR)**. Creatinine is an endogenous byproduct of muscle metabolism that is freely filtered by the glomerulus. Because it is not reabsorbed and only minimally secreted by the tubules, the amount of creatinine cleared from the plasma per unit of time closely approximates the volume of plasma filtered by the glomeruli. Therefore, it serves as a reliable marker for overall glomerular function and the kidney's ability to filter waste. **2. Why Other Options are Incorrect:** * **Afferent loop pressure:** This refers to hemodynamic pressures within the renal arterioles. While pressure affects GFR, creatinine clearance measures the *result* of filtration, not the specific hydrostatic pressure of the afferent vessel. * **Tubular function:** Tubular function is assessed using tests for concentration/dilution (e.g., Water Deprivation Test) or specific solute handling (e.g., Fractional Excretion of Sodium - $FE_{Na}$). Since creatinine is primarily handled by filtration rather than tubular processing, it is a poor indicator of tubular health. **3. High-Yield Clinical Pearls for NEET-PG:** * **Overestimation:** $C_{Cr}$ slightly **overestimates** GFR (by ~10-20%) because a small amount of creatinine is secreted by the proximal convoluted tubule. * **Gold Standard:** The "Gold Standard" for measuring GFR is **Inulin clearance**, as it is filtered but neither secreted nor reabsorbed. However, it is not used clinically because it is exogenous and requires continuous infusion. * **Cockcroft-Gault Formula:** Often used to estimate $C_{Cr}$ based on age, weight, and serum creatinine. Remember to multiply by **0.85 for females**. * **Inverse Relationship:** Serum creatinine has an inverse, non-linear relationship with GFR. A doubling of serum creatinine suggests a 50% reduction in GFR.

Question 314: Which substance(s) is/are freely filtered across glomerular capillaries?

A. Glucose (Correct Answer)
B. Albumin
C. Thyroxin
D. Creatinine

Explanation: **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.

Question 315: What is the major source of ammonia in the kidney?

A. Glutamate
B. Glutamine (Correct Answer)
C. alpha-ketoglutarate
D. Alanine

Explanation: **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.

Question 316: Ultrafiltrate passes through all filtration barriers except?

A. Endothelial fenestra
B. Basement membrane
C. Fenestra in podocytes (Correct Answer)
D. Filtration slit between podocytes

Explanation: 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.

Question 317: Which statement is false regarding the ascending limb of the loop of Henle?

A. It is highly permeable to sodium.
B. A defect in the Na+K+2Cl- channel leads to Pendred syndrome. (Correct Answer)
C. It is permeable to magnesium.
D. It is a part of the countercurrent multiplier system.

Explanation: ### Explanation **Why Option B is the Correct Answer (The False Statement):** A defect in the **Na⁺-K⁺-2Cl⁻ (NKCC2) cotransporter** in the Thick Ascending Limb (TAL) of the loop of Henle leads to **Bartter Syndrome**, not Pendred syndrome. Bartter syndrome is characterized by salt wasting, hypokalemia, and metabolic alkalosis. **Pendred syndrome**, conversely, is a genetic disorder caused by mutations in the *SLC26A4* gene (encoding the protein pendrin), leading to sensorineural hearing loss and thyroid goiter. **Analysis of Other Options:** * **Option A (True):** The TAL is highly permeable to sodium due to the active transport mediated by the NKCC2 transporter. It is often called the "diluting segment" because it reabsorbs solutes while remaining **impermeable to water**. * **Option C (True):** The TAL is a major site for paracellular reabsorption of divalent cations like **Magnesium (Mg²⁺)** and Calcium (Ca²⁺). This is driven by the lumen-positive potential created by the back-leak of potassium through ROMK channels. * **Option D (True):** The ascending limb is the "active" component of the **countercurrent multiplier**. By pumping out NaCl into the medullary interstitium without water, it establishes the osmotic gradient necessary for urine concentration. **High-Yield Clinical Pearls for NEET-PG:** * **Loop Diuretics (Furosemide):** Act by inhibiting the NKCC2 transporter in the TAL. * **Gitelman Syndrome:** A defect in the NaCl cotransporter (NCC) in the Distal Convoluted Tubule (DCT). * **Liddle Syndrome:** Gain-of-function mutation in ENaC channels (Collecting Duct), leading to hypertension and hypokalemia. * **Countercurrent Multiplier vs. Exchanger:** The Loop of Henle is the *multiplier*; the Vasa Recta is the *exchanger*.

Question 318: In the proximal tubule, bicarbonate (HCO3-) reabsorption occurs via which mechanism?

A. Sodium-chloride cotransporter
B. Sodium-potassium-2-chloride cotransporter
C. Sodium-hydrogen exchanger (Correct Answer)
D. Sodium-potassium exchanger

Explanation: **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 319: A negatively charged molecule is filtered with more difficulty compared to a positively charged one because why?

A. The filtering membrane has negatively charged sialoproteins. (Correct Answer)
B. Negatively charged molecules are inherently larger.
C. The filtering membrane has positively charged proteins.
D. Urine is typically acidic.

Explanation: ### Explanation The glomerular filtration barrier (GFB) is a highly selective interface composed of three layers: the fenestrated endothelium, the glomerular basement membrane (GBM), and the podocyte slit diaphragms. **1. Why Option A is Correct:** The GFB acts as both a **size barrier** and a **charge barrier**. The endothelial glycocalyx, the GBM, and the podocytes are coated with polyanionic glycoproteins, primarily **sialoproteins** (like podocalyxin) and heparan sulfate proteoglycans. These components carry a strong **negative charge**. According to Coulomb’s Law, like charges repel; therefore, negatively charged molecules (anions) face electrostatic repulsion and are filtered with much greater difficulty than neutral or positively charged molecules (cations) of the same molecular radius. **2. Why Other Options are Incorrect:** * **Option B:** Molecular charge and molecular size are independent physical properties. A small negative ion may be filtered more easily than a massive positive protein, but for molecules of equal size, the charge is the deciding factor. * **Option C:** If the membrane had positively charged proteins, it would facilitate the loss of albumin (which is negatively charged), leading to massive proteinuria. * **Option D:** While urine pH can affect the ionization of certain drugs, it does not determine the intrinsic permeability of the glomerular capillary wall. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Albumin Paradox:** Albumin has a molecular radius of ~3.6 nm, which is small enough to pass through the size barrier, but it is restricted almost entirely due to its **negative charge**. * **Minimal Change Disease (MCD):** The primary pathology is the **loss of negative charges** (sialoproteins) on the GFB. This results in "selective proteinuria" (mainly albuminuria) because the charge barrier is gone while the size barrier remains relatively intact. * **Dextran Studies:** Experimental studies using neutral, anionic, and cationic dextrans are the classic proof of the GFB's charge selectivity.

Question 320: Which characteristic allows a particle to pass easily through the glomerular membrane, assuming it has the same diameter as another particle?

A. Positive charge (Correct Answer)
B. Negative charge
C. Neutral charge
D. Charge has no relation to glomerular filtration

Explanation: ### Explanation The glomerular filtration barrier is a highly selective interface composed of three layers: the fenestrated endothelium, the glomerular basement membrane (GBM), and the podocyte slit diaphragms. Its selectivity is determined by two factors: **size** and **electrical charge**. **1. Why Positive Charge is Correct:** The glomerular membrane is lined with **negatively charged glycoproteins** (primarily **sialoglycoproteins** and **heparan sulfate**). These polyanions create an electrostatic field that repels other negatively charged molecules while attracting positively charged ones. Therefore, for two particles of the same diameter, a **cationic (positively charged)** particle will be filtered more readily than a neutral or anionic one because it is electrostatically pulled through the barrier. **2. Why Other Options are Incorrect:** * **Negative Charge:** Anionic molecules (like albumin) experience **electrostatic repulsion** from the negatively charged GBM and podocytes. This is why albumin, despite being small enough to fit through the pores, is restricted from filtration. * **Neutral Charge:** Neutral molecules are filtered based solely on their size (molecular radius). While they pass more easily than negative particles, they do not benefit from the electrostatic attraction that positive particles do. * **Charge has no relation:** This is incorrect as the "charge barrier" is a fundamental physiological principle of renal filtration. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Minimal Change Disease (MCD):** The primary pathology is the **loss of negative charges** on the glomerular basement membrane. This results in "selective proteinuria" (mainly albuminuria) because the charge barrier is gone, even though the size barrier remains intact. * **Dextran Studies:** Experimental studies using dextran (a polymer that can be manufactured with different charges) prove that for any given molecular radius, **Cationic Dextran > Neutral Dextran > Anionic Dextran** in terms of filterability. * **Albumin:** It has a molecular radius of ~3.6 nm. While the glomerular pore is ~4 nm, albumin is not filtered primarily due to its strong negative charge.

Practice by Chapter

Renal Blood Flow and Glomerular Filtration

Practice Questions

Tubular Reabsorption and Secretion

Practice Questions

Concentration and Dilution of Urine

Practice Questions

Acid-Base Regulation by the Kidneys

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Regulation

Practice Questions

Calcium and Phosphate Handling

Practice Questions

Micturition Physiology

Practice Questions

Renal Function Tests

Practice Questions

Integrative Responses to Fluid Challenges

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free