Renal Physiology — MCQs

As the glomerular filtrate passes through the uriniferous tubule, ions and water are exchanged (actively and passively) with the renal interstitium. These exchanges result in the filtrate being isotonic, hypotonic, or hypertonic relative to blood plasma. What is the tonicity of the filtrate in Bowman's space?

Q540

Renin is secreted from which of the following?

Renal Physiology Indian Medical PG Practice Questions and MCQs

Question 531: Which mechanism is primarily responsible for the transport of glucose in renal tubular cells?

A. Facilitated diffusion
B. Glucose diffusion
C. Sodium antiport
D. Sodium-glucose cotransport (Correct Answer)

Explanation: ***Sodium-glucose cotransport*** - Glucose reabsorption in the renal tubules, particularly in the **proximal tubule**, occurs primarily via **secondary active transport** involving **sodium-glucose cotransporters (SGLTs)**. - SGLT proteins use the **sodium concentration gradient** (maintained by the Na+/K+-ATPase on the basolateral membrane) to move glucose against its concentration gradient from the tubular lumen into the cell. *Glucose diffusion* - While passive diffusion may play a minor role, it is insufficient to reabsorb the large amounts of **filtered glucose** - Diffusion would lead to significant **glucose loss in urine**, even at normal blood glucose levels. *Sodium antiport* - Antiport systems move two different ions or molecules in **opposite directions** across a membrane. - While present in renal cells, sodium antiport mechanisms are not the primary means of **glucose reabsorption**; rather, glucose transport is mostly symport. *Facilitated diffusion* - Facilitated diffusion involves carrier proteins (like **GLUT transporters**) that move molecules down their **concentration gradient**. - While GLUT transporters are present on the **basolateral membrane** of tubular cells to move glucose into the interstitium, they are not the primary mechanism for glucose uptake from the tubular lumen, which occurs against a concentration gradient.

Question 532: Hormone necessary for water and sodium balance is

A. Progesterone
B. Cortisol
C. Estrogen
D. Aldosterone (Correct Answer)

Explanation: ***Aldosterone*** - **Aldosterone** is a mineralocorticoid hormone that plays a crucial role in regulating **sodium and water balance** by acting on the kidneys to increase sodium reabsorption and potassium excretion. - This increased sodium reabsorption leads to increased water reabsorption, thereby maintaining **blood volume** and **blood pressure**. *Progesterone* - **Progesterone** is primarily involved in the **menstrual cycle, pregnancy, and embryonic development**. - While it can have some diuretic effects, its primary role is not in the direct daily regulation of **water and sodium balance**. *Cortisol* - **Cortisol** is a glucocorticoid hormone involved in stress response, metabolism, and immune function. - While it has some minor mineralocorticoid activity at high concentrations, it is **not the primary hormone** responsible for water and sodium balance. *Estrogen* - **Estrogen** is a sex hormone primarily involved in the development of female secondary sexual characteristics and reproductive processes. - It can cause **fluid retention** in some cases, but it does not have a direct or primary role in the regulation of **water and sodium balance** like aldosterone.

Question 533: Which radionuclide is best suited for measuring glomerular filtration rate (GFR)?

A. DTPA (Correct Answer)
B. DMSA
C. EDTA
D. Ortho-Iodohippurate

Explanation: ***DTPA*** - **DTPA (⁹⁹ᵐTc-DTPA)** is cleared almost exclusively by **glomerular filtration**, making it an excellent marker for GFR measurement. - Its rapid plasma clearance correlates well with **inulin clearance**, which is the gold standard for GFR. - **⁹⁹ᵐTc labeling** provides superior imaging properties, ready availability from generators, and optimal gamma energy for detection. *DMSA* - **DMSA (⁹⁹ᵐTc-dimercaptosuccinic acid)** primarily binds to the **renal cortex** and is used to assess renal parenchymal function and anatomy. - It does not accurately reflect GFR because it is mainly handled by **tubular uptake**, not glomerular filtration. *Ortho-Iodohippurate* - **Ortho-Iodohippurate (¹³¹I-OIH or ⁹⁹ᵐTc-MAG3)** is predominantly cleared by **tubular secretion**, making it a good measure of **effective renal plasma flow (ERPF)**. - While it provides information on renal function, it is not suitable for direct GFR assessment. *EDTA* - **EDTA (⁵¹Cr-EDTA)** is also cleared by glomerular filtration and can accurately measure GFR, particularly used in Europe. - However, **DTPA is preferred** due to the advantages of **⁹⁹ᵐTc labeling** (better availability, imaging properties, and lower radiation dose) compared to **⁵¹Cr labeling**. - Both are valid GFR markers, but DTPA is more commonly used in routine clinical practice.

Question 534: Consider the following conditions: 1) Diabetes mellitus 2) Heart failure 3) Hyperuricemia 4) Chronic kidney disease. Which of the following conditions result in solute diuresis?

A. Diabetes mellitus, hyperuricemia and chronic kidney disease
B. Diabetes mellitus only (Correct Answer)
C. Heart failure, hyperuricemia and chronic kidney disease
D. Heart failure, diabetes mellitus and hyperuricemia

Explanation: ***Diabetes mellitus only*** - **Diabetes mellitus** causes **osmotic (solute) diuresis** due to glycosuria when blood glucose exceeds the renal threshold (~180 mg/dL) - High glucose levels in the renal tubules exceed the reabsorptive capacity of glucose transporters (SGLT2 in proximal tubule) - The non-reabsorbed glucose acts as an osmotic agent, obligating water excretion and causing polyuria - This is the classic example of solute diuresis in clinical medicine *Diabetes mellitus, hyperuricemia and chronic kidney disease* - While **diabetes mellitus** does cause solute diuresis, **hyperuricemia** (elevated serum uric acid) does NOT cause solute diuresis - Hyperuricemia may result FROM diuretic use but is not itself a cause of diuresis - **Chronic kidney disease** does not primarily cause solute diuresis; advanced CKD may show "osmotic diuresis per nephron" as an adaptive mechanism, but CKD itself is not a cause of solute diuresis *Heart failure, hyperuricemia and chronic kidney disease* - **Heart failure** causes sodium and water **retention** (not diuresis) due to reduced effective arterial blood volume and activation of RAAS and sympathetic nervous system - **Hyperuricemia** and **chronic kidney disease** are not primary causes of solute diuresis - This option incorrectly includes conditions that do not cause solute diuresis *Heart failure, diabetes mellitus and hyperuricemia* - While **diabetes mellitus** causes solute diuresis, **heart failure** leads to fluid retention rather than diuresis - **Hyperuricemia** does not cause solute diuresis - This option mixes one correct condition with two incorrect ones

Question 535: Inulin clearance closely resembles which of the following?

A. Renal plasma flow
B. Creatinine clearance
C. PAH clearance
D. Glomerular filtration rate (GFR) (Correct Answer)

Explanation: ***Glomerular filtration rate (GFR)*** - **Inulin** is a polysaccharide that is **freely filtered** by the glomerulus and is neither secreted nor reabsorbed by the renal tubules. - This property makes its clearance a nearly perfect measure of the **glomerular filtration rate (GFR)**. *Renal plasma flow* - **Renal plasma flow (RPF)** is typically measured using substances like **PAH (para-aminohippurate)**, which are both filtered and secreted, showing high extraction by the kidneys. - Inulin clearance significantly **underestimates RPF** since it only reflects glomerular filtration. *Creatinine clearance* - **Creatinine clearance** is commonly used as an approximation of GFR, but it tends to **slightly overestimate true GFR** because creatinine is also secreted by the renal tubules. - Unlike inulin, creatinine is an **endogenous substance**, making its measurement easier in clinical practice, but less precise than inulin for GFR. *PAH clearance* - **PAH (para-aminohippurate) clearance** is used to measure **effective renal plasma flow (ERPF)** because it is both filtered and actively secreted, leading to nearly complete extraction from the plasma in a single pass through the kidneys. - Inulin, being only filtered, measures **glomerular filtration**, which is a distinct—and much smaller—component of renal function than total plasma flow.

Question 536: In response to an increase in glomerular filtration rate, the proximal tubule and the loop of Henle demonstrate an increase in the rate of sodium reabsorption. This phenomenon is called?

A. Glomerulotubular balance (Correct Answer)
B. Autoregulation of renal blood flow
C. Mineralocorticoid escape (response to aldosterone)
D. Saturation of tubular transport capacity

Explanation: ***Glomerulotubular balance*** - This mechanism ensures that a relatively constant fraction of the filtered load of sodium and water is reabsorbed by the proximal tubule and loop of Henle, despite variations in GFR. - An increased GFR leads to an increased filtered load of sodium, which in turn stimulates greater reabsorption in these segments, maintaining constancy. *Autoregulation of renal blood flow* - This refers to the kidney's intrinsic ability to maintain a relatively constant **glomerular filtration rate (GFR)** and **renal blood flow (RBF)** despite fluctuations in systemic arterial pressure. - It involves mechanisms like the **myogenic response** and **tubuloglomerular feedback** at the afferent arteriole, but does not directly describe the proportional reabsorption of sodium. *Mineralocorticoid escape (response to aldosterone)* - This phenomenon describes the **limited effectiveness of aldosterone** in causing sustained sodium retention or edema due to compensatory mechanisms. - It involves an increase in atrial natriuretic peptide (ANP), pressure natriuresis, and decreased proximal tubule reabsorption in response to chronic aldosterone excess. *Saturation of tubular transport capacity* - This occurs when the amount of a substance filtered into the tubules exceeds the maximum rate at which the tubules can reabsorb or secrete it, leading to its excretion in urine. - This concept describes the **limit of tubular transport**, not the proportional adjustment in reabsorption in response to GFR changes.

Question 537: How is the fractional excretion of sodium (FENa) calculated?

A. [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100 (Correct Answer)
B. [(Serum Na × Urine Cr) / (Urine Na × Serum Cr)] × 100
C. [(Serum Na × Serum Cr) / (Urine Na × Urine Cr)] × 100
D. [(Urine Na × Urine Cr) / (Serum Na × Serum Cr)] × 100

Explanation: ***[(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100*** - This formula correctly represents the fractional excretion of sodium, which is the percentage of filtered sodium that is excreted in the urine. - It essentially compares the renal clearance of sodium to the renal clearance of creatinine, which serves as a proxy for the **glomerular filtration rate**. *[(Serum Na × Urine Cr) / (Urine Na × Serum Cr)] × 100* - This formula inverts the correct ratio, leading to an inaccurate calculation of **sodium excretion**. - It would disproportionately weigh serum sodium and urine creatinine, components that are not directly used in that product in the correct formula. *[(Serum Na × Serum Cr) / (Urine Na × Urine Cr)] × 100* - This formula incorrectly mixes serum and urine concentrations in a way that does not reflect the concept of **fractional excretion**. - It forms products of serum concentrations in the numerator and urine concentrations in the denominator, which is not physiologically meaningful for FENa. *[(Urine Na × Urine Cr) / (Serum Na × Serum Cr)] × 100* - This formula is also incorrect as it groups urine parameters in the numerator and serum parameters in the denominator, which does not represent the ratio of **sodium clearance** to **creatinine clearance**. - The correct formula isolates the urinary excretion of sodium relative to the amount filtered.

Question 538: Two particles have the same diameter and molecular weight. Which factor would LEAST likely affect their passage through the glomerular filtration barrier?

A. Positively charged particles
B. Charge does not affect passage
C. Negatively charged particles
D. Either charge can pass (Correct Answer)

Explanation: ***Either charge can pass*** - When two particles have the **same diameter and molecular weight**, charge becomes the primary differentiating factor for glomerular filtration - However, the question asks which factor would **LEAST affect passage** - both positively and negatively charged particles **CAN pass through** the glomerular filtration barrier, though at different rates - While charge significantly affects the **rate** of filtration, it does not create an absolute barrier - this makes "either charge can pass" the most accurate answer as it represents the least absolute effect on passage capability - The **glomerular basement membrane** contains negatively charged **heparan sulfate proteoglycans**, creating charge selectivity but not complete exclusion *Positively charged particles* - **Positively charged particles** filter **more readily** through the glomerular barrier due to electrostatic attraction to the negatively charged basement membrane - This represents a significant effect on passage rate, making charge a major factor for these particles - The enhanced filtration of cationic molecules is a well-established principle in renal physiology *Negatively charged particles* - **Negatively charged particles** are **relatively restricted** from passing through due to electrostatic repulsion from the negatively charged basement membrane - This significant hindrance to filtration demonstrates that charge strongly affects passage for anionic molecules - Albumin (negatively charged) is largely excluded from filtration partly due to charge repulsion *Charge does not affect passage* - This statement is **physiologically incorrect** - **Charge is a critical determinant** of glomerular permeability, along with size and shape - The charge selectivity of the glomerular barrier is fundamental to renal physiology and prevents excessive protein loss

Question 539: As the glomerular filtrate passes through the uriniferous tubule, ions and water are exchanged (actively and passively) with the renal interstitium. These exchanges result in the filtrate being isotonic, hypotonic, or hypertonic relative to blood plasma. What is the tonicity of the filtrate in Bowman's space?

A. Isotonic (Correct Answer)
B. Hypotonic
C. Hyperosmotic
D. Hypertonic

Explanation: ***Isotonic*** - The glomerular filtrate that collects in **Bowman's space** is formed by the **ultrafiltration of plasma** across the glomerular capillaries. - This process allows water and small solutes to pass through, but restricts large proteins and blood cells, resulting in a filtrate with an **osmolarity essentially identical to that of blood plasma** (~300 mOsm/L). *Hypotonic* - A hypotonic solution has a **lower solute concentration** compared to blood plasma. - While filtrate becomes hypotonic in the **ascending limb of the loop of Henle** due to selective solute reabsorption without water reabsorption, it is not hypotonic in Bowman's space. *Hyperosmotic* - **Hyperosmotic** (or hypertonic) refers to a solution with a **higher solute concentration** compared to blood plasma. - The filtrate becomes hyperosmotic in the **descending limb of the loop of Henle** (due to water reabsorption) and in the **collecting duct** under ADH influence, but not in Bowman's space. *Hypertonic* - A hypertonic solution has a **higher solute concentration** compared to blood plasma. - This describes the filtrate in certain portions of the nephron (deep medullary collecting duct can reach >1200 mOsm/L), but the initial filtrate in Bowman's space is isotonic to plasma.

Question 540: Renin is secreted from which of the following?

A. JG cells (Correct Answer)
B. Mesangial cells
C. Macula densa cells
D. Lacis cells

Explanation: ***JG cells*** - **Juxtaglomerular (JG) cells** are specialized smooth muscle cells located in the **afferent arteriole** of the kidney glomerulus. - They synthesize, store, and release **renin** in response to decreased renal perfusion pressure, sympathetic stimulation, or decreased sodium delivery to the macula densa. *Mesangial cells* - **Mesangial cells** are connective tissue cells located within the glomerulus, between the glomerular capillaries. - They provide structural support for the glomerular capillaries and play a role in regulating glomerular filtration, but they **do not secrete renin**. *Macula densa cells* - **Macula densa cells** are specialized epithelial cells in the distal convoluted tubule that sense **sodium chloride concentration** in the filtrate. - While they are part of the juxtaglomerular apparatus and influence renin release, they **do not directly secrete renin** themselves; instead, they signal JG cells. *Lacis cells* - **Lacis cells** (also known as extraglomerular mesangial cells) are located in the angle between the afferent and efferent arterioles, adjacent to the macula densa and JG cells. - Their exact function is not fully understood, but they are thought to facilitate communication within the **juxtaglomerular apparatus** and provide structural support, not renin secretion.

Practice by Chapter

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