Renal Physiology Practice Questions

Q: Glucose is reabsorbed in the proximal tubule via symport with which ion?

Na+. **Explanation:** Glucose reabsorption in the proximal convoluted tubule (PCT) occurs via **Secondary Active Transport**. This process is mediated by **Sodium-Glucose Co-transporters (SGLT)**, specifically SGLT2 (early PCT) and SGLT1 (late PCT). **Why Na+ is correct:** The process is driven by the electrochemical gradient created by the **Na+-K+ ATPase pump** on the basolateral membrane, which keeps intracellular sodium levels low. This gradient allows sodium to move down its concentration gradient from the tubular lumen into the cell. Glucose "hitchhikes" with sodium against its own concentration gradient via a **symport** mechanism. Once inside the cell, glucose exits the basolateral membrane into the blood via facilitated diffusion through **GLUT2** (or GLUT1). **Why other options are incorrect:** * **K+ (Potassium):** Potassium is primarily secreted or reabsorbed through specific channels and exchangers (like the Na-K-2Cl symporter in the Loop of Henle), but it does not drive glucose transport. * **Ca++ (Calcium):** Calcium reabsorption in the PCT is largely passive (paracellular) and follows water and sodium; it is not coupled with glucose. * **Cl- (Chloride):** Chloride is reabsorbed primarily via paracellular pathways or in exchange for other anions in the later segments of the PCT. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT2 Inhibitors (e.g., Dapagliflozin):** A class of drugs used in Diabetes Mellitus that block glucose reabsorption in the PCT, leading to glucosuria. * **Renal Threshold for Glucose:** Approximately **180 mg/dL**. When blood glucose exceeds this, SGLT transporters become saturated ($T_mG$), and glucose appears in the urine. * **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, and phosphates in the urine.

Q: What will be the plasma clearance value of glucose in a patient with diabetes mellitus?

Greater than zero. **Explanation:** The plasma clearance of a substance is the volume of plasma from which that substance is completely removed by the kidneys per unit time. **1. Why the Correct Answer is Right:** In a healthy individual, the plasma clearance of glucose is **zero** because all filtered glucose is reabsorbed in the proximal convoluted tubule (PCT) via SGLT-2 transporters. However, in **Diabetes Mellitus**, the filtered load of glucose often exceeds the **Renal Threshold** (approx. 180 mg/dL) and the **Tubular Transport Maximum ($T_mG$)** (approx. 375 mg/min in men). Once the transporters are saturated, the excess glucose cannot be reabsorbed and is excreted in the urine (glycosuria). Since glucose is now being cleared from the plasma into the urine, the clearance value becomes **greater than zero**. **2. Why the Incorrect Options are Wrong:** * **Option A (Zero):** This is true for healthy individuals, but not for diabetics with hyperglycemia exceeding the renal threshold. * **Option B (Equal to Inulin):** Inulin clearance represents the GFR because it is filtered but neither reabsorbed nor secreted. For glucose clearance to equal inulin, zero reabsorption would have to occur, which is not the case even in diabetes. * **Option C (Greater than PAH):** PAH clearance represents Renal Plasma Flow (RPF) because it is filtered and maximally secreted. No substance can have a clearance higher than the RPF. **3. Clinical Pearls for NEET-PG:** * **Renal Threshold for Glucose:** ~180 mg/dL (venous blood). * **Splay:** The curve representing the difference between the theoretical and actual renal threshold due to the heterogeneity of nephrons and low affinity of transporters. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** These drugs intentionally lower the renal threshold to induce glycosuria, thereby increasing glucose clearance to treat Type 2 Diabetes.

Q: In clinical practice, which substance's plasma clearance value is most commonly used to estimate Glomerular Filtration Rate (GFR)?

Creatinine. **Explanation:** The Glomerular Filtration Rate (GFR) is the best overall index of kidney function. To measure GFR, a substance must be freely filtered at the glomerulus and neither reabsorbed nor secreted by the renal tubules. **Why Creatinine is the Correct Answer:** In clinical practice, **Creatinine** is the most commonly used endogenous marker for estimating GFR. It is a metabolic byproduct of muscle (creatine phosphate) produced at a relatively constant rate. While it is slightly secreted by the proximal tubule (leading to a 10-20% overestimation of GFR), its convenience—requiring no intravenous infusion—makes it the practical gold standard for routine clinical assessment via the **Creatinine Clearance** test or eGFR formulas (like MDRD or CKD-EPI). **Analysis of Incorrect Options:** * **A. Inulin:** This is the **"Gold Standard"** for measuring GFR because it is perfectly filtered and neither secreted nor reabsorbed. However, it is an exogenous polysaccharide requiring continuous IV infusion and frequent blood sampling, making it impractical for routine clinical use. * **B. Para-aminohippuric acid (PAH):** PAH is filtered and extensively secreted, such that it is almost completely cleared from the blood in one pass. Therefore, it is used to measure **Renal Plasma Flow (RPF)**, not GFR. * **C. Glucose:** Under normal physiological conditions, glucose is filtered but completely reabsorbed in the proximal tubule (Clearance = 0). It cannot be used to measure GFR. **High-Yield Clinical Pearls for NEET-PG:** * **Filtration Fraction (FF):** GFR / RPF (Normal ≈ 20%). * **Creatinine Secretion:** Drugs like **Cimetidine** and **Trimethoprim** inhibit tubular secretion of creatinine, falsely elevating serum creatinine levels without changing the actual GFR. * **Cystatin C:** An emerging endogenous marker that is not affected by muscle mass or diet, unlike creatinine.

Q: In free water clearance, where is free water generated?

Ascending limb of loop of Henle. **Explanation:** Free water clearance ($C_{H2O}$) represents the volume of blood plasma that is cleared of solute-free water per unit of time. The generation of free water is fundamentally dependent on the **separation of solutes from water**, a process that occurs in the "diluting segments" of the nephron. **Why Option C is Correct:** The **Thick Ascending Limb (TAL)** of the loop of Henle is the primary site for free water generation. This segment is **impermeable to water** but actively reabsorbs solutes (Na⁺, K⁺, and Cl⁻) via the **NKCC2 cotransporter**. As solutes leave the tubule without water following them, the tubular fluid becomes dilute (hypotonic), effectively "generating" free water that can either be excreted (in diuresis) or reabsorbed (in antidiuresis). **Why Other Options are Incorrect:** * **Option A (PCT):** In the PCT, water and solutes are reabsorbed **isotonically** (in equal proportions). Since the tubular fluid remains isosmotic to plasma, no free water is generated here. * **Option B (Descending Limb):** This segment is highly permeable to water but impermeable to solutes. Water leaves the tubule, making the fluid hypertonic; thus, it concentrates the urine rather than generating free water. **High-Yield Facts for NEET-PG:** * **Diluting Segments:** Both the TAL and the Early Distal Convoluted Tubule are considered diluting segments because they are impermeable to water. * **Loop Diuretics:** Drugs like Furosemide act on the TAL (NKCC2). By inhibiting solute reabsorption, they **abolish the corticomedullary gradient** and prevent the generation of free water. * **Formula:** $C_{H2O} = V - C_{osm}$ (where $V$ is urine flow rate and $C_{osm}$ is osmolar clearance). * Positive $C_{H2O}$: Dilute urine (e.g., Diabetes Insipidus). * Negative $C_{H2O}$: Concentrated urine (e.g., SIADH).

Q: In osmotic diuresis, what occurs?

Decrease in the reabsorption of NaCl. ### Explanation **Mechanism of Osmotic Diuresis** Osmotic diuresis occurs when non-reabsorbable solutes (e.g., glucose in diabetes mellitus or administered mannitol) remain in the renal tubule. These solutes exert an **osmotic pressure** that opposes the reabsorption of water. As water is retained in the lumen, the concentration of sodium ($Na^+$) in the tubular fluid decreases. This creates a concentration gradient that favors the back-leak of $Na^+$ into the lumen and **decreases the net reabsorption of NaCl** and water, primarily in the Proximal Convoluted Tubule (PCT) and the Loop of Henle. **Analysis of Options** * **Option D (Correct):** The presence of osmotic agents increases tubular flow rate and reduces the contact time for transporters. The dilution of luminal $Na^+$ further inhibits its active and passive reabsorption, leading to increased excretion of both water and electrolytes. * **Option A & B (Incorrect):** Osmotic diuresis typically does not significantly increase Renal Blood Flow (RBF) or Glomerular Filtration Rate (GFR). In fact, severe diuresis can lead to hypovolemia, which may eventually **decrease** GFR due to compensatory vasoconstriction. * **Option C (Incorrect):** While the *total amount* (mass) of NaCl excreted increases, the **concentration** of NaCl in the urine is usually lower than in normal urine because the water loss is proportionately greater than the solute loss (dilutional effect). **NEET-PG High-Yield Pearls** * **Mannitol** is the classic osmotic diuretic; it acts mainly on the **Proximal Tubule** and the **Descending limb of the Loop of Henle**. * **Clinical Sign:** Osmotic diuresis leads to a "washout" of the medullary osmotic gradient, impairing the kidney's ability to concentrate urine even in the presence of ADH. * **Key Difference:** Unlike water diuresis (low ADH), osmotic diuresis is characterized by a high urine osmolality and significant electrolyte loss.

Question 1

A substance is present in a concentration of 2 mg% in the afferent arteriole and zero mg% in the efferent arteriole. What is true about this substance?

Accepted Answer

It is freely filtered at the glomerulus.

Answer

It is secreted in the cortical nephron.

Answer

It is absorbed in the proximal convoluted tubule.

Answer

It is impermeable in the loop of Henle.

Question 2

Net filtration pressure of the glomerulus increases when which of the following occurs?

Accepted Answer

Increase in hydrostatic pressure of the glomerular capillary

Answer

Increase in oncotic pressure of the glomerular capillary

Answer

Increase in hydrostatic pressure of Bowman's space

Answer

All of the above

Question 3

Glucose is reabsorbed in the proximal tubule via symport with which ion?

Accepted Answer

Na+

Answer

K+

Answer

Ca++

Answer

Cl-

Question 4

Glucosuria occurs when the venous blood glucose concentration exceeds which of the following values?

Accepted Answer

10 mg/dL

Answer

100 mg/dL

Answer

180 mg/dL

Answer

18 mg/dL

Question 5

What will be the plasma clearance value of glucose in a patient with diabetes mellitus?

Accepted Answer

Greater than zero

Answer

Zero

Answer

Equal to that of inulin clearance

Answer

Greater than that of PAH clearance

Question 6

In clinical practice, which substance's plasma clearance value is most commonly used to estimate Glomerular Filtration Rate (GFR)?

Accepted Answer

Creatinine

Answer

Inulin

Answer

Para-aminohippuric acid (PAH)

Answer

Glucose

Question 7

A substance is present in a concentration of 2 mg/dL in the afferent arteriole and zero mg/dL in the efferent arteriole. Which of the following is true about the substance?

Accepted Answer

It is freely filtered in the glomerulus.

Answer

It is secreted in the cortical nephron.

Answer

It is absorbed in the proximal convoluted tubule.

Answer

It is impermeable in the loop of Henle.

Question 8

In free water clearance, where is free water generated?

Accepted Answer

Ascending limb of loop of Henle

Answer

Proximal convoluted tubule (PCT)

Answer

Descending limb of loop of Henle

Answer

All of the above

Question 9

How can the reabsorption of iso-osmolar fluid from the glomerular filtrate be increased?

Accepted Answer

Increased filtered load

Answer

Increased peritubular capillary pressure

Answer

Decreased osmotic pressure in peritubular capillaries

Answer

Increased corticomedullary osmotic gradient

Question 10

In osmotic diuresis, what occurs?

Accepted Answer

Decrease in the reabsorption of NaCl

Answer

Increase in renal blood flow

Answer

Increase in glomerular filtration rate

Answer

Increase in NaCl concentration in urine

Renal Physiology — MCQs

Renal Physiology — MCQs

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