Renal Physiology Practice Questions

Q: During pregnancy, what happens to the Glomerular Filtration Rate (GFR)?

Increases. **Explanation:** **Why the correct answer is right:** During pregnancy, the Glomerular Filtration Rate (GFR) **increases significantly (by approximately 40-50%)**. This physiological adaptation begins as early as the first trimester. The primary driver is a massive increase in **Renal Plasma Flow (RPF)**, which rises by nearly 50-80%. This occurs due to systemic vasodilation mediated by hormones like **Relaxin** and Nitric Oxide, which decrease systemic vascular resistance. Consequently, both afferent and efferent arterioles dilate (with afferent dilation being more prominent), leading to increased blood flow to the glomerulus and a subsequent rise in GFR. **Why incorrect options are wrong:** * **B & C:** Decreased or unchanged GFR is pathological in pregnancy. A failure of GFR to rise may indicate underlying renal disease or pre-eclampsia. * **D:** While GFR may be slightly higher in multiple gestations due to greater plasma volume expansion, the increase is a fundamental physiological change occurring in **all** normal pregnancies, not just multiple ones. **High-Yield Clinical Pearls for NEET-PG:** * **Serum Creatinine & BUN:** Because GFR increases, the clearance of waste products increases. Therefore, normal serum creatinine and BUN levels are **lower** in pregnancy (Normal Cr: 0.4–0.8 mg/dL). A "normal" non-pregnant creatinine level (e.g., 1.1 mg/dL) may actually indicate renal impairment in a pregnant patient. * **Glucosuria:** The increased GFR often exceeds the proximal tubule's capacity to reabsorb glucose (**TmG**), leading to physiological glucosuria even in the absence of diabetes. * **Peak:** GFR peaks at the end of the first trimester and remains elevated until term.

Q: If the plasma concentration of a freely filterable substance is 2 mg/mL, GFR is 100 mL/min, urine concentration of the substance is 10 mg/mL, and urine flow rate is 5 mL/min, what can be concluded about the kidney tubules?

Reabsorbed 150 mg/min. To solve this problem, we must compare the **Filtered Load** of the substance with its **Excretion Rate**. ### 1. Calculation * **Filtered Load:** This is the amount of substance filtered at the glomerulus per minute. * Formula: $GFR \times \text{Plasma Concentration } (P_x)$ * Calculation: $100\text{ mL/min} \times 2\text{ mg/mL} = \mathbf{200\text{ mg/min}}$ * **Excretion Rate:** This is the amount of substance actually leaving the body in urine per minute. * Formula: $\text{Urine Flow Rate } (V) \times \text{Urine Concentration } (U_x)$ * Calculation: $5\text{ mL/min} \times 10\text{ mg/mL} = \mathbf{50\text{ mg/min}}$ ### 2. Interpretation Since the **Filtered Load (200 mg/min)** is greater than the **Excretion Rate (50 mg/min)**, it means that 150 mg of the substance was "lost" during its passage through the tubules. Therefore, the tubules must have **reabsorbed** the difference. * **Net Transport = Filtered Load – Excretion Rate** * $200 - 50 = \mathbf{150\text{ mg/min (Reabsorbed)}}$ ### 3. Why Incorrect Options are Wrong * **B (Reabsorbed 200 mg/min):** This would imply an excretion rate of zero (complete reabsorption, like glucose in a healthy individual). * **C & D (Secreted):** Secretion occurs only when the Excretion Rate is **greater** than the Filtered Load (e.g., PAH or Creatinine). Here, the excretion is much lower than the filtration. ### 4. NEET-PG High-Yield Pearls * **Clearance Ratio:** If $C_x / C_{\text{inulin}} 1$, the substance is secreted. * **Freely Filterable:** This term implies the substance's concentration in the Bowman’s space equals its concentration in the plasma (no protein binding). * **Glucose:** At normal plasma levels, filtered load = reabsorption rate (Excretion = 0). Glucosuria occurs only when the filtered load exceeds the $T_m$ (Transport Maximum) of SGLT transporters.

Q: Where in the kidney does active reabsorption of sodium ions occur?

All of the above. **Explanation:** Sodium (Na+) reabsorption is the primary driver of renal function, with approximately 99% of filtered sodium being reabsorbed along the nephron. While the **Proximal Convoluted Tubule (PCT)** is the site of bulk reabsorption (65%), active transport mechanisms exist throughout the distal segments. * **Ascending limb of Henle:** The Thick Ascending Limb (TAL) reabsorbs ~25% of filtered sodium via the **NKCC2 cotransporter** (Sodium-Potassium-2 Chloride). This is an active process (secondary active transport) driven by the basolateral Na+/K+ ATPase pump. * **Distal tubule:** The early distal tubule reabsorbs sodium via the **NCC (Sodium-Chloride) symporter**. * **Collecting duct:** Principal cells in the late distal tubule and collecting ducts reabsorb sodium through **ENaC (Epithelial Sodium Channels)**. This step is highly regulated by **Aldosterone**, making it the "fine-tuning" phase of sodium balance. **Why "All of the above" is correct:** Active sodium reabsorption occurs in every segment mentioned. The energy for this transport is universally derived from the **basolateral Na+/K+ ATPase pump**, which maintains a low intracellular sodium concentration, creating the electrochemical gradient necessary for sodium entry from the tubular lumen. **High-Yield Clinical Pearls for NEET-PG:** 1. **Loop Diuretics (Furosemide):** Inhibit the NKCC2 transporter in the Thick Ascending Limb. 2. **Thiazides:** Inhibit the NCC symporter in the Distal Convoluted Tubule. 3. **Potassium-sparing diuretics (Amiloride/Spironolactone):** Act on the ENaC channels or Aldosterone receptors in the Collecting Duct. 4. **Descending limb of Henle:** This is the only segment where sodium is **not** actively reabsorbed (it is permeable to water but impermeable to solutes).

Q: Renin is synthesized as a large preprohormone. Where is it secreted from?

Juxtaglomerular apparatus. **Explanation:** **1. Why the Correct Answer is Right:** Renin is a proteolytic enzyme synthesized, stored, and secreted by the **Juxtaglomerular (JG) cells**. These are specialized, granular modified smooth muscle cells located primarily in the afferent arteriole (and to a lesser extent, the efferent arteriole) at the point where it contacts the Distal Convoluted Tubule. The JG cells, along with the Macula Densa and Lacis cells, form the **Juxtaglomerular Apparatus (JGA)**. Renin is synthesized as *preprorenin*, cleaved to *prorenin*, and finally stored as active *renin* in secretory granules, released in response to low perfusion pressure, sympathetic stimulation, or decreased NaCl delivery. **2. Why the Incorrect Options are Wrong:** * **A. PCT:** The PCT is primarily responsible for the bulk reabsorption of water, electrolytes, and glucose. It does not possess endocrine secretory functions for renin. * **B. DCT:** While the **Macula Densa** is located in the initial part of the DCT, these cells act as *sensors* for NaCl concentration; they do not secrete renin themselves but signal the JG cells to do so. * **C. Collecting Duct:** This segment is involved in the final concentration of urine under the influence of ADH and aldosterone; it is not the site of renin synthesis. **3. NEET-PG High-Yield Clinical Pearls:** * **Rate-Limiting Step:** Renin secretion is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS). * **Stimuli for Release:** 1) Decreased renal perfusion pressure (Baroreceptor mechanism), 2) Increased sympathetic activity (via **$\beta_1$ receptors**), 3) Decreased NaCl delivery to Macula Densa. * **Goldblatt Kidney:** A classic experimental model of hypertension caused by renal artery stenosis, leading to excessive renin release from the JGA. * **Inhibitor:** Plasma Renin Activity (PRA) is decreased by Beta-blockers and NSAIDs.

Q: A patient has a blood glucose level of 200 mg/dL and a GFR of 90 mL/min. The transport maximum for glucose is 150 mg/min. What is the amount of glucose excreted per minute?

30 mg/min. To solve this problem, we must apply the fundamental physiological principle of renal glucose handling: **Excretion = Filtered Load – Reabsorption.** ### 1. Calculation of the Correct Answer (A) * **Filtered Load:** This is the amount of glucose filtered through the glomerulus per minute. * *Formula:* Filtered Load = Plasma Glucose Concentration × GFR * *Calculation:* 200 mg/dL (which is 2 mg/mL) × 90 mL/min = **180 mg/min**. * **Reabsorption:** Glucose is reabsorbed in the proximal convoluted tubule via SGLT transporters. However, this process has a limit known as the **Transport Maximum ($T_m$)**. * In this patient, the $T_m$ is **150 mg/min**. Since the filtered load (180 mg/min) exceeds the $T_m$, the kidney can only reabsorb 150 mg/min. * **Excretion:** * *Calculation:* 180 mg/min (Filtered) – 150 mg/min (Reabsorbed) = **30 mg/min**. ### 2. Why Other Options are Incorrect * **B (40 mg/min):** This would occur if the filtered load was 190 mg/min or the $T_m$ was 140 mg/min. * **C (20 mg/min):** This would occur if the filtered load was 170 mg/min. * **D (50 mg/min):** This would occur if the filtered load was 200 mg/min. ### 3. Clinical Pearls for NEET-PG * **Renal Threshold:** The plasma glucose concentration at which glucose first appears in the urine (glycosuria) is typically **180 mg/dL**. * **Splay:** The curve of glucose excretion is not a sharp angle; it is rounded. This "splay" occurs because not all nephrons have the same $T_m$ and due to the low affinity of transporters near saturation. * **Transporters:** Glucose is reabsorbed by **SGLT-2** (90%, early PCT) and **SGLT-1** (10%, late PCT). SGLT-2 inhibitors (e.g., Dapagliflozin) are now key drugs in managing Diabetes Mellitus.

Q: The Water Deprivation Test is used to assess which of the following aspects of renal function?

Distal Tubular Function. ### Explanation **1. Why Distal Tubular Function is Correct:** The Water Deprivation Test (also known as the Miller-Moses test) is the gold standard for evaluating the kidney's **concentrating ability**. This process is primarily mediated by the **Distal Convoluted Tubule (DCT) and Collecting Ducts**. Under conditions of water deprivation, plasma osmolality rises, triggering the release of **Antidiuretic Hormone (ADH)** from the posterior pituitary. ADH acts on the V2 receptors in the distal segments of the nephron to insert aquaporin-2 channels, allowing water reabsorption. A normal response is the production of highly concentrated urine. If the distal tubules fail to respond to ADH (Nephrogenic Diabetes Insipidus) or if ADH is absent (Central Diabetes Insipidus), the urine remains dilute despite dehydration. **2. Why Other Options are Incorrect:** * **A. Glomerular Function:** This is assessed by the **Glomerular Filtration Rate (GFR)** using clearance studies (e.g., Inulin or Creatinine clearance). * **B. Proximal Tubular Function:** This is evaluated by measuring the reabsorption of substances like glucose (TmG), amino acids, or bicarbonate, often impaired in Fanconi Syndrome. * **D. Renal Plasma Flow (RPF):** This is measured using the clearance of **Para-aminohippuric acid (PAH)**, as it is both filtered and secreted. **3. Clinical Pearls for NEET-PG:** * **Primary Polydipsia vs. DI:** The test helps differentiate psychogenic polydipsia from Diabetes Insipidus (DI). * **Desmopressin Challenge:** If urine osmolality does not rise after water deprivation, **Exogenous Vasopressin (Desmopressin)** is administered. * *Rise in Osmolality (>50%):* Central DI. * *No/Minimal Rise ( 800 mOsm/kg during the test.

Q: Calculate the filtration fraction if the Glomerular Filtration Rate (GFR) is 125 ml/min and the Renal Plasma Flow is 625 ml/min.

20%. **Explanation** **1. Understanding the Correct Answer (B: 20%)** The **Filtration Fraction (FF)** represents the proportion of the renal plasma flow that is actually filtered across the glomerular capillaries into the Bowman’s space. It is a critical indicator of renal efficiency and hemodynamics. The formula to calculate Filtration Fraction is: $$\text{FF} = \frac{\text{GFR}}{\text{RPF}} \times 100$$ Plugging in the values from the question: $$\text{FF} = \frac{125 \text{ ml/min}}{625 \text{ ml/min}} = 0.20 \text{ or } 20\%$$ This means that 20% of the plasma entering the kidneys is filtered, while the remaining 80% leaves the glomerulus via the efferent arteriole to become peritubular capillary flow. **2. Analysis of Incorrect Options** * **A (5%) & D (10%):** These values are too low for a healthy adult. Such low fractions may be seen in conditions where GFR is severely compromised relative to blood flow, such as acute glomerulonephritis. * **C (50%):** This is physiologically improbable. A filtration fraction this high would excessively increase the oncotic pressure in the peritubular capillaries, potentially leading to sluggish blood flow and impaired renal function. **3. NEET-PG High-Yield Pearls** * **Normal Range:** The physiological FF is typically **19–20%**. * **Effect of Efferent Vasoconstriction:** Angiotensin II preferentially constricts the efferent arteriole. This increases glomerular hydrostatic pressure, thereby **increasing the FF** (even if RPF decreases). * **Clinical Significance:** An increased FF is often seen in **Congestive Heart Failure (CHF)** because the body attempts to maintain GFR despite a drop in total renal blood flow. * **RPF vs. RBF:** Remember that RPF is only the plasma component. If the question provides Renal Blood Flow (RBF), you must first calculate RPF using the formula: $RPF = RBF \times (1 - \text{Hematocrit})$.

Q: Administration of two litres of normal saline over 4 hours will stimulate the secretion of which of the following?

Atrial natriuretic peptide. **Explanation:** The administration of two litres of normal saline (an isotonic solution) leads to **volume expansion** of the extracellular fluid (ECF) compartment. **Why Atrial Natriuretic Peptide (ANP) is correct:** When ECF volume increases, there is a corresponding increase in venous return to the heart. This causes **stretching of the atrial myocytes**. In response to this mechanical stretch, the atria secrete ANP. ANP acts as a potent hypotensive agent by promoting **natriuresis** (sodium excretion) and **diuresis** (water excretion) through the dilation of afferent arterioles and constriction of efferent arterioles in the kidney. It also inhibits the Renin-Angiotensin-Aldosterone System (RAAS), helping the body return to a normovolemic state. **Why the other options are incorrect:** * **Interleukin-2 (IL-2) and Tumor Necrosis Factor-alpha (TNF-α):** These are inflammatory cytokines. Their secretion is triggered by immune responses, infections, or tissue injury, not by simple volume expansion. * **Prostaglandins:** While renal prostaglandins (like PGE2) play a role in maintaining renal blood flow, they are typically synthesized in response to vasoconstrictors (like Angiotensin II) to prevent excessive ischemia, rather than as a primary response to volume loading. **Clinical Pearls for NEET-PG:** * **Brain Natriuretic Peptide (BNP):** Similar to ANP but primarily secreted by the **ventricles** in response to pressure overload or volume expansion. It is a key clinical marker for heart failure. * **Isotonic Saline Effect:** Infusing 0.9% NaCl increases ECF volume but does not change intracellular fluid (ICF) volume because there is no osmotic gradient created. * **ANP Mechanism:** It works via the **cGMP** second messenger system (guanylyl cyclase).

Question 1

During pregnancy, what happens to the Glomerular Filtration Rate (GFR)?

Accepted Answer

Increases

Answer

Decreases

Answer

Remains unchanged

Answer

Increases only in multiple gestations

Question 2

In which of the following ways does aldosterone interact with renal tubular cells?

Accepted Answer

Increases translocation of ENaCs from the cytoplasm to the cell membrane

Answer

Binds to a steroid receptor inside the nucleus of renal tubular cells

Answer

Binds to ENaCs on the cell membrane and increases sodium transport

Answer

Binds to G-protein coupled receptors on the cell surface

Question 3

If the plasma concentration of a freely filterable substance is 2 mg/mL, GFR is 100 mL/min, urine concentration of the substance is 10 mg/mL, and urine flow rate is 5 mL/min, what can be concluded about the kidney tubules?

Accepted Answer

Reabsorbed 150 mg/min

Answer

Reabsorbed 200 mg/min

Answer

Secreted 50 mg/min

Answer

Secreted 150 mg/min

Question 4

Where in the kidney does active reabsorption of sodium ions occur?

Accepted Answer

All of the above

Answer

Collecting duct

Answer

Distal tubule

Answer

Ascending limb of Henle

Question 5

Renin is synthesized as a large preprohormone. Where is it secreted from?

Accepted Answer

Juxtaglomerular apparatus

Answer

Proximal Convoluted Tubule (PCT)

Answer

Distal Convoluted Tubule (DCT)

Answer

Collecting duct

Question 6

A patient has a blood glucose level of 200 mg/dL and a GFR of 90 mL/min. The transport maximum for glucose is 150 mg/min. What is the amount of glucose excreted per minute?

Accepted Answer

30 mg/min

Answer

40 mg/min

Answer

20 mg/min

Answer

50 mg/min

Question 7

The Water Deprivation Test is used to assess which of the following aspects of renal function?

Accepted Answer

Distal Tubular Function

Answer

Glomerular Function

Answer

Proximal Tubular Function

Answer

Renal Plasma Flow

Question 8

Stimulation of which receptor produces renin?

Accepted Answer

Beta 1

Answer

Alpha 1

Answer

Alpha 2

Answer

Beta 2

Question 9

Calculate the filtration fraction if the Glomerular Filtration Rate (GFR) is 125 ml/min and the Renal Plasma Flow is 625 ml/min.

Accepted Answer

20%

Answer

5%

Answer

50%

Answer

10%

Question 10

Administration of two litres of normal saline over 4 hours will stimulate the secretion of which of the following?

Accepted Answer

Atrial natriuretic peptide

Answer

Interleukin-2

Answer

Tumor Necrosis Factor-alpha

Answer

Prostaglandins

Renal Physiology — MCQs

Renal Physiology — MCQs

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