Renal Physiology Practice Questions

Q: Which of the following actions does aldosterone secretion from the adrenal gland regulate in the kidney?

Exchange of Na+ for K+ in the distal convoluted tubule. **Explanation:** **Mechanism of Action:** Aldosterone is a mineralocorticoid synthesized in the *zona glomerulosa* of the adrenal cortex. Its primary site of action is the **Principal cells (P cells)** of the **late distal convoluted tubule (DCT)** and the **collecting ducts**. Aldosterone acts by binding to intracellular mineralocorticoid receptors, leading to the upregulation of: 1. **Basolateral Na+/K+ ATPase pumps:** These create a low intracellular Na+ concentration. 2. **Apical ENaC (Epithelial Sodium Channels):** These allow Na+ to be reabsorbed from the tubular lumen into the cell. 3. **Apical ROMK (Renal Outer Medullary Potassium) channels:** These facilitate the secretion of K+ into the lumen. Therefore, the net effect is the **reabsorption of Na+** (from the lumen to the blood) in exchange for the **secretion of K+** (from the blood to the lumen). This makes Option B the correct physiological description. **Analysis of Incorrect Options:** * **Options C & D:** These are incorrect because the **Proximal Convoluted Tubule (PCT)** is not the site of aldosterone action. The PCT is responsible for bulk reabsorption (65% of Na+ and water) independent of aldosterone. * **Option A:** This describes the exchange in the wrong direction. Aldosterone does not exchange K+ (reabsorption) for Na+ (secretion); it does the opposite to maintain electrolyte balance and blood pressure. **NEET-PG High-Yield Pearls:** * **Conn’s Syndrome:** Primary hyperaldosteronism leads to the triad of **Hypertension, Hypokalemia, and Metabolic Alkalosis**. * **Spironolactone/Eplerenone:** These are potassium-sparing diuretics that act as competitive antagonists to aldosterone receptors. * **Intercalated Cells:** While aldosterone acts on P cells for Na+/K+ exchange, it also acts on **Alpha-intercalated cells** to secrete H+ ions via H+-ATPase, explaining why excess aldosterone causes alkalosis.

Q: In a normal adult, the desire for micturition is typically felt when approximately what volume of urine has collected in the bladder?

300-400 cc. **Explanation:** The process of micturition is regulated by the **micturition reflex**, which is initiated by stretch receptors in the bladder wall (specifically the detrusor muscle). 1. **Why B is correct:** In a healthy adult, the bladder can accommodate small volumes with little change in intravesical pressure due to "plasticity." However, once the volume reaches **300-400 cc**, the tension in the bladder wall increases sufficiently to trigger the sensory nerves. These impulses travel to the sacral segments of the spinal cord and the micturition center in the pons, resulting in the conscious **desire to void**. 2. **Why other options are incorrect:** * **A (100-200 cc):** While the very first *vague* sensation of bladder filling may occur around 150 cc, it is usually not perceived as a distinct "desire to micturate" in a normal physiological state. * **C (600-800 cc):** At this volume, the bladder is significantly overdistended. The sensation shifts from a "desire to void" to **pain** and extreme urgency. * **D (1000 cc):** This represents the anatomical capacity of the bladder. At this stage, the internal sphincter may fail, leading to overflow incontinence or potential bladder rupture. **High-Yield NEET-PG Pearls:** * **First sensation of filling:** ~150 ml. * **First desire to void:** ~300 ml. * **Painful distension:** >600 ml. * **Nerve Supply:** The **Pelvic nerve** (Parasympathetic, S2-S4) is the primary nerve for the micturition reflex (contraction of detrusor). The **Pudendal nerve** (S2-S4) provides voluntary control over the external sphincter. * **Cystometrogram:** The graph plotting intravesical pressure against volume; the sharp rise in pressure (Law of Laplace) occurs after the bladder exceeds its functional capacity.

Q: Glucose transport and reabsorption in the kidneys is inhibited by which of the following?

Phlorhizin. **Explanation:** The correct answer is **Phlorhizin**. **1. Why Phlorhizin is correct:** Glucose reabsorption in the kidney occurs primarily in the proximal convoluted tubule (PCT) via **SGLT2** (90%) and **SGLT1** (10%) transporters [1]. **Phlorhizin** is a naturally occurring glucoside (found in the bark of fruit trees) that acts as a **non-selective, competitive inhibitor of both SGLT1 and SGLT2**. By blocking these transporters, it prevents the reabsorption of filtered glucose, leading to glycosuria and a reduction in blood glucose levels. While not used clinically due to poor absorption and GI side effects, it served as the prototype for the modern "Gliflozin" class (e.g., Dapagliflozin) [2]. **2. Why other options are incorrect:** * **Tetrodotoxin:** A potent neurotoxin found in pufferfish. It acts by blocking **voltage-gated sodium channels**, inhibiting action potentials in nerve and muscle cells. It has no effect on glucose transport. * **GLUT:** These are a family of **uniporters** (facilitated diffusion) responsible for moving glucose across cell membranes (e.g., GLUT2 on the basolateral membrane of the PCT). They are the *mediators* of transport, not inhibitors. * **SGLT:** Sodium-Glucose Linked Transporters are the **targets** for inhibition. SGLT proteins facilitate the active transport of glucose; they do not inhibit it. **3. High-Yield NEET-PG Clinical Pearls:** * **SGLT2 Inhibitors (Gliflozins):** These are the clinical descendants of phlorhizin. They are now first-line agents for Type 2 Diabetes, especially in patients with **Heart Failure (HFrEF)** and **Chronic Kidney Disease (CKD)** due to their cardio-protective and reno-protective effects [2]. * **Location:** SGLT2 is located in the **S1 segment** of the PCT, while SGLT1 is in the **S3 segment**. * **Side Effects:** The most common side effect of SGLT2 inhibitors is **genital mycotic infections** (candidiasis) due to increased glucose in the urine.

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.

Question 1

Given known values for glomerular filtration rate (GFR), the maximum rate for tubular reabsorption (Tm), and plasma concentration (P) for a reabsorbed substance, what is the amount of the substance appearing in the urine per unit time when the plasma concentration is above that for saturation of reabsorption?

Accepted Answer

P (GFR) - Tm

Answer

Tm (GFR) - P

Answer

Tm (GFR) + P

Answer

Tm - P (GFR)

Question 2

Which of the following actions does aldosterone secretion from the adrenal gland regulate in the kidney?

Accepted Answer

Exchange of Na+ for K+ in the distal convoluted tubule

Answer

Exchange of K+ for Na+ in the distal convoluted tubule

Answer

Exchange of K+ for Na+ in the proximal convoluted tubule

Answer

Exchange of Na+ for K+ in the proximal convoluted tubule

Question 3

In a normal adult, the desire for micturition is typically felt when approximately what volume of urine has collected in the bladder?

Accepted Answer

300-400 cc

Answer

100-200 cc

Answer

600-800 cc

Answer

1000 cc

Question 4

In a patient, if the administration of vasopressin does not increase the osmolality of urine, what does it indicate?

Accepted Answer

Renal hyposensitivity towards ADH

Answer

Antidiuretic hormone (ADH) deficiency

Answer

Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)

Answer

Psychogenic polydipsia

Question 5

Glucose transport and reabsorption in the kidneys is inhibited by which of the following?

Accepted Answer

Phlorhizin

Answer

Tetrodotoxin

Answer

GLUT

Answer

SGLT

Question 6

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 7

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 8

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 9

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 10

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

Renal Physiology — MCQs

Renal Physiology — MCQs

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