Renal Physiology Practice Questions

Q: Which substance is secreted by the proximal convoluted tubule (PCT)?

Hydrogen ions. ### Explanation The **Proximal Convoluted Tubule (PCT)** is the most metabolically active segment of the nephron, responsible for both bulk reabsorption and selective secretion. **Why Hydrogen ions (H⁺) is correct:** The PCT plays a vital role in acid-base balance. It secretes **Hydrogen ions** into the tubular lumen, primarily via the **Sodium-Hydrogen Exchanger 3 (NHE3)**. This secretion is coupled with the reabsorption of filtered bicarbonate ($HCO_3^-$). For every $H^+$ ion secreted, one $HCO_3^-$ molecule is returned to the blood, making the PCT the primary site for maintaining the body's alkali reserve. **Why the other options are incorrect:** * **Sodium (A):** Sodium is **reabsorbed** (not secreted) in the PCT. Approximately 65% of filtered sodium is reabsorbed here through various symporters and antiporters. * **Glucose (B) & Amino acids (D):** These are essential nutrients that are **100% reabsorbed** in the early PCT under normal physiological conditions. Glucose is reabsorbed via **SGLT-2** (and some SGLT-1), while amino acids use specific sodium-coupled transporters. Their presence in urine (glycosuria/aminoaciduria) indicates a pathological state or exceeding the renal threshold. **High-Yield NEET-PG Pearls:** * **Secreted substances in PCT:** $H^+$, Ammonium ($NH_4^+$), Creatinine, and organic acids/bases (e.g., Penicillin, PAH, Urate). * **Carbonic Anhydrase:** The enzyme essential for $H^+$ secretion and $HCO_3^-$ reabsorption in the PCT. **Acetazolamide** inhibits this enzyme, leading to proximal renal tubular acidosis. * **Fanconi Syndrome:** A clinical condition resulting from generalized dysfunction of the PCT, leading to the wasting of glucose, amino acids, phosphate, and bicarbonate in the urine.

Q: Which part of the nephron is the dilution segment?

Ascending limb of the Loop of Henle. ### Explanation The correct answer is **A. Ascending limb of the Loop of Henle**. **Why it is the Diluting Segment:** The Thick Ascending Limb (TAL) of the Loop of Henle is termed the "diluting segment" because it is **impermeable to water** but actively reabsorbs solutes (Na⁺, K⁺, and Cl⁻) via the **NKCC2 transporter**. As electrolytes are pumped out into the medullary interstitium while water remains trapped in the tubule, the tubular fluid becomes increasingly dilute (hypotonic) relative to plasma. By the time the fluid reaches the distal convoluted tubule, its osmolarity drops to approximately 100 mOsm/L. **Analysis of Incorrect Options:** * **B. Descending limb of the Loop of Henle:** This is the "concentrating segment." It is highly permeable to water but impermeable to solutes. Water leaves the tubule via osmosis, making the tubular fluid hypertonic. * **C. Collecting tubule:** While dilution can occur here in the absence of ADH, it is primarily responsible for the final concentration of urine under the influence of ADH (Vasopressin). * **D. Proximal tubule:** Reabsorption here is **isostatic**. Both water and solutes are reabsorbed in equal proportions, so the tubular fluid remains isotonic (300 mOsm/L) to plasma. **NEET-PG High-Yield Pearls:** * **NKCC2 Transporter:** This is the target of **Loop Diuretics** (e.g., Furosemide). By inhibiting this transporter, these drugs abolish the corticomedullary gradient, preventing both dilution and concentration of urine. * **Bartter Syndrome:** A genetic defect in the NKCC2 transporter or associated channels in the TAL, mimicking chronic loop diuretic use. * **Countercurrent Multiplier:** The TAL provides the "single effect" that drives the countercurrent multiplier system, essential for the kidney's ability to concentrate urine.

Q: A patient's urine is collected for 2 hours, and the total volume is 600 milliliters during this time. Her urine osmolarity is 150 mOsm/L, and her plasma osmolarity is 300 mOsm/L. What is her free water clearance?

+2.5 ml/min. ### Explanation **1. Understanding the Concept** Free water clearance ($C_{H_2O}$) represents the volume of solute-free water excreted by the kidneys per unit of time. It is calculated using the formula: $$C_{H_2O} = V - C_{osm}$$ Where: * **$V$ (Urine Flow Rate):** Total volume / Time = $600\text{ ml} / 120\text{ min} = \mathbf{5\text{ ml/min}}$. * **$C_{osm}$ (Osmolar Clearance):** $\frac{U_{osm} \times V}{P_{osm}} = \frac{150 \times 5}{300} = \mathbf{2.5\text{ ml/min}}$. **Calculation:** $C_{H_2O} = 5\text{ ml/min} - 2.5\text{ ml/min} = \mathbf{+2.5\text{ ml/min}}$. Since the urine is dilute ($U_{osm} < P_{osm}$), the kidney is "clearing" excess water from the plasma, resulting in a positive value. **2. Analysis of Options** * **Option B (Correct):** As calculated, the net excretion of solute-free water is 2.5 ml/min. * **Option A (+5.0 ml/min):** This is the total urine flow rate ($V$), not the free water clearance. It ignores the volume required to excrete the solutes. * **Option C (0.0 ml/min):** This occurs when urine is isosthenuric ($U_{osm} = P_{osm}$), meaning no free water is being gained or lost. * **Option D (-2.5 ml/min):** A negative value (free water reabsorption) occurs only when urine is concentrated ($U_{osm} > P_{osm}$), typically under the influence of high ADH levels. **3. NEET-PG High-Yield Pearls** * **Positive $C_{H_2O}$:** Seen in states of water excess, Diabetes Insipidus, or use of loop diuretics (where urine is dilute). * **Negative $C_{H_2O}$ ($T^c_{H_2O}$):** Seen in dehydration or SIADH, where the kidneys conserve water and produce concentrated urine. * **Site of Action:** Free water is primarily generated in the **thick ascending limb of the Loop of Henle** (the "diluting segment") where solutes are reabsorbed without water.

Q: Where is SGLT 2 primarily found?

None of the above. **Explanation:** The correct answer is **None of the above** because **SGLT 2 (Sodium-Glucose Cotransporter 2)** is primarily and almost exclusively located in the **Kidney**, specifically in the **S1 and S2 segments of the Proximal Convoluted Tubule (PCT)** [2]. It is responsible for reabsorbing approximately 90% of the filtered glucose from the tubular fluid back into the bloodstream [1]. **Analysis of Options:** * **A. Liver:** The liver primarily utilizes **GLUT 2** (a glucose transporter) for the bidirectional movement of glucose; it does not express SGLT 2 [2]. * **B. Intestine:** The primary sodium-glucose cotransporter in the small intestine is **SGLT 1**, which is responsible for the absorption of glucose and galactose from the dietary lumen [2]. (Note: SGLT 1 is also found in the S3 segment of the renal PCT, accounting for the remaining 10% of glucose reabsorption) [2]. * **C. Spleen:** The spleen does not play a significant role in active glucose transport via SGLT proteins. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT 2 Inhibitors (Gliflozins):** Drugs like Dapagliflozin and Empagliflozin inhibit SGLT 2 in the PCT, leading to glucosuria. They are used in managing Type 2 Diabetes, Heart Failure, and Chronic Kidney Disease. * **SGLT 1 vs. SGLT 2:** Remember "1" is for the Gut (and 10% Kidney), and "2" is for the Kidney (90%). * **Fanconi Syndrome:** A generalized dysfunction of the PCT where SGLT 2 function is impaired along with the reabsorption of amino acids, uric acid, and phosphates.

Q: What is the normal Transtubular potassium gradient (TTKG) in hypokalemia?

< 3-4. **Explanation:** The **Transtubular Potassium Gradient (TTKG)** is a clinical tool used to estimate the conservation of potassium in the cortical collecting duct (CCD). It reflects the activity of aldosterone and the kidney's response to potassium levels. **1. Why Option A is Correct:** In a healthy physiological state, if a patient has **hypokalemia**, the kidneys should respond by maximizing potassium reabsorption to conserve it. This results in a very low concentration of potassium in the tubular fluid relative to the plasma. A **TTKG 3-4 in the presence of hypokalemia, it suggests **renal potassium wasting** (e.g., primary hyperaldosteronism or diuretic use). **2. Why Other Options are Incorrect:** * **Options B, C, and D:** These values represent higher gradients. A TTKG **> 7-10** is considered normal in a **hyperkalemic** patient, indicating that the kidneys are appropriately secreting excess potassium. If these high values are seen during hypokalemia, it indicates an inappropriate renal loss of potassium. **Clinical Pearls for NEET-PG:** * **Formula:** $TTKG = \frac{[K^+]_{urine} / [K^+]_{plasma}}{Osm_{urine} / Osm_{plasma}}$ * **Prerequisites:** For TTKG to be accurate, the urine must be concentrated ($Osm_{urine} > Osm_{plasma}$) and urine sodium should be $> 25 \text{ mEq/L}$ to ensure adequate delivery to the distal tubule. * **High-Yield Fact:** TTKG is primarily used to differentiate between **extra-renal** (TTKG 3-4) causes of hypokalemia.

Question 1

A 50-year-old female goes into shock with a blood pressure of 50 mmHg. Which of the following concerning her Glomerular Filtration Rate (GFR) is most likely?

Accepted Answer

GFR will virtually cease.

Answer

Myogenic autoregulation will keep GFR constant.

Answer

Angiotensin II release will maintain normal GFR.

Answer

The macula densa will cause constriction of the afferent arteriole to maintain GFR.

Question 2

Which substance is secreted by the proximal convoluted tubule (PCT)?

Accepted Answer

Hydrogen ions

Answer

Sodium

Answer

Glucose

Answer

Amino acids

Question 3

Which part of the nephron is the dilution segment?

Accepted Answer

Ascending limb of the Loop of Henle

Answer

Descending limb of the Loop of Henle

Answer

Collecting tubule

Answer

Proximal tubule

Question 4

In the kidney, which of the following statements is true?

Accepted Answer

The osmolarity of the medullary interstitium is greater than that of plasma.

Answer

The osmolarity of tubular fluid in the PCT gradually increases.

Answer

The osmolarity of fluid in the DCT is more than in Bowman's capsule.

Answer

Fluid coming out of the descending limb of the loop of Henle is hypotonic.

Question 5

A patient's urine is collected for 2 hours, and the total volume is 600 milliliters during this time. Her urine osmolarity is 150 mOsm/L, and her plasma osmolarity is 300 mOsm/L. What is her free water clearance?

Accepted Answer

+2.5 ml/min

Answer

+5.0 ml/min

Answer

0.0 ml/min

Answer

-2.5 ml/min

Question 6

A 25-year-old woman presents with nausea and vomiting for 2 days. Her physical examination is normal except for a postural drop in blood pressure from 110/80 mm Hg supine to 90/80 mm Hg standing. Her serum electrolytes are sodium 130 mEq/L, potassium 3 mEq/L, chloride 90 mEq/L, bicarbonate 30 mEq/L, urea 50 mg/dL, and creatinine 0.8 mg/dL. Which of the following electrolytes is most likely to be filtered through the glomerulus but unaffected by tubular secretion?

Accepted Answer

Urea

Answer

Potassium

Answer

Sodium

Answer

Bicarbonate

Question 7

Where is SGLT 2 primarily found?

Accepted Answer

None of the above

Answer

Liver

Answer

Intestine

Answer

Spleen

Question 8

Sympathectomy of the kidney results in which of the following responses?

Accepted Answer

Increased urine output

Answer

No change to the glomerular filtration rate, nor to the urine output

Answer

Decreased urine output

Answer

Reduction of glomerular filtration rate and decreased urinary output

Question 9

Which of the following are actions of angiotensin II?

Accepted Answer

Systemic vasodilation

Answer

Systemic vasoconstriction

Answer

Renal vasodilatation

Answer

Reabsorption of Na+ in the proximal renal tubule

Question 10

What is the normal Transtubular potassium gradient (TTKG) in hypokalemia?

Accepted Answer

< 3-4

Answer

> 6-7

Answer

> 9-10

Answer

> 10-15

Renal Physiology — MCQs

Renal Physiology — MCQs

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