Renal Physiology Practice Questions

Q: What is the primary function of Antidiuretic Hormone (ADH)?

Promotes water reabsorption. **Explanation:** **Antidiuretic Hormone (ADH)**, also known as Vasopressin, is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and secreted by the posterior pituitary. Its primary role is the regulation of plasma osmolality. **Why Option A is Correct:** ADH acts on the **V2 receptors** located on the basolateral membrane of the **principal cells** in the late distal tubule and collecting ducts. This triggers a cAMP-mediated signaling pathway that leads to the insertion of **Aquaporin-2 (AQP2)** water channels into the apical membrane. This increases the permeability of the collecting duct to water, allowing water to be reabsorbed down the osmotic gradient into the hypertonic renal medullary interstitium. **Why Other Options are Incorrect:** * **Option B:** While ADH results in decreased water excretion, this is a *consequence* of its primary physiological action, which is active reabsorption. * **Option C:** Sodium reabsorption is primarily regulated by **Aldosterone** (acting on ENaC channels). While ADH can have minor effects on sodium transporters (NKCC2), it is not its primary function. * **Option D:** Potassium secretion is a function of Aldosterone and tubular flow rates, not ADH. **High-Yield Clinical Pearls for NEET-PG:** * **V1 Receptors:** Located on vascular smooth muscle; cause vasoconstriction (hence the name Vasopressin). * **Diabetes Insipidus (DI):** Deficiency of ADH (Central DI) or resistance to ADH (Nephrogenic DI) leads to polyuria and dilute urine. * **SIADH:** Excessive ADH secretion leads to water retention, dilutional hyponatremia, and concentrated urine. * **Urea Recycling:** ADH also increases the permeability of the medullary collecting duct to urea (via UT-A1 transporters), which helps maintain the medullary osmotic gradient.

Q: Ethanol increases urine output by which of the following mechanisms?

Suppressing ADH secretion. ### Explanation **Mechanism of Action** The correct answer is **B. Suppressing ADH secretion**. Ethanol (alcohol) acts as a potent diuretic primarily by inhibiting the release of **Antidiuretic Hormone (ADH)**, also known as Vasopressin, from the posterior pituitary gland. Under normal conditions, ADH acts on the V2 receptors of the collecting ducts to insert **aquaporin-2 channels**, allowing water reabsorption. By suppressing ADH, ethanol prevents these channels from being inserted, leading to decreased water permeability in the distal nephron. This results in the excretion of a large volume of dilute urine (**water diuresis**). **Analysis of Incorrect Options** * **A. Inhibiting Na+ reabsorption:** This is the mechanism of action for diuretics like Furosemide or Thiazides (saluretics). Ethanol does not primarily interfere with sodium transport. * **C. Increasing glomerular filtration rate (GFR):** While minor changes in GFR can occur due to systemic effects, it is not the primary or consistent mechanism behind ethanol-induced diuresis. * **D. Suppressing H+ secretion:** This would affect acid-base balance (potentially causing acidosis) but does not directly lead to increased urine volume. **High-Yield Clinical Pearls for NEET-PG** * **Site of Action:** Ethanol acts centrally on the **supraoptic and paraventricular nuclei** of the hypothalamus. * **The "Breaking the Seal" Phenomenon:** The rapid onset of diuresis after alcohol consumption is due to the immediate drop in plasma ADH levels. * **Dehydration:** The diuresis caused by ethanol is often out of proportion to the fluid intake, leading to the cellular dehydration characteristic of a "hangover." * **Other ADH Inhibitors:** Cold exposure and ANP (Atrial Natriuretic Peptide) also inhibit ADH secretion.

Q: What is the most potent stimulus for renin release?

Hypotension. **Explanation:** Renin release is the rate-limiting step of the Renin-Angiotensin-Aldosterone System (RAAS). While multiple factors stimulate the juxtaglomerular (JG) cells, **hypotension** (specifically a decrease in renal perfusion pressure) is considered the most potent and direct stimulus. 1. **Why Hypotension is correct:** The JG cells in the afferent arteriole act as **intrarenal baroreceptors**. A drop in mean arterial pressure directly stretches these cells less, triggering an immediate and robust release of renin to restore systemic blood pressure and maintain glomerular filtration rate (GFR). 2. **Analysis of Incorrect Options:** * **Sympathetic stimulation (Option A):** Activated via $\beta_1$-adrenergic receptors on JG cells during stress or exercise. While significant, it is often a secondary response to systemic hypotension. * **Decreased NaCl in the DCT (Option B):** This is sensed by the **Macula Densa**. When NaCl delivery drops (indicating low GFR), the macula densa signals the JG cells to release renin. This is a vital feedback mechanism but is generally considered less "potent" than direct baroreceptor activation. * **Prostacyclin (Option C):** Prostaglandins ($PGE_2$ and $PGI_2$) act as local paracrine signaling molecules that stimulate renin release, but they usually act as mediators for the macula densa pathway rather than primary stimuli. **Clinical Pearls for NEET-PG:** * **Location:** Renin is synthesized and stored in the **modified smooth muscle cells** (JG cells) of the **afferent arteriole**. * **Inhibitors:** Renin release is inhibited by **Atrial Natriuretic Peptide (ANP)** and high levels of Angiotensin II (negative feedback). * **Mnemonic:** Stimuli for Renin = **"The 3 Bs"** — **B**aroreceptors (Pressure), **B**ath (NaCl content at Macula Densa), and **B**eta-1 receptors (Sympathetic).

Q: Filtration fraction can be best determined by which pair of substances?

PAH and Inulin. **Explanation:** **Filtration Fraction (FF)** is the ratio of the Glomerular Filtration Rate (GFR) to the Renal Plasma Flow (RPF). It represents the fraction of plasma entering the kidneys that is actually filtered across the glomerular capillaries into the Bowman’s space. The formula is: **FF = GFR / RPF** 1. **Why Option B is Correct:** * **Inulin** is the gold standard for measuring **GFR** because it is freely filtered but neither reabsorbed nor secreted by the renal tubules. * **Para-aminohippuric acid (PAH)** is used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and actively secreted, resulting in almost complete clearance from the plasma in a single pass through the kidney. * Therefore, the ratio of Inulin clearance to PAH clearance provides the most accurate determination of the Filtration Fraction. 2. **Why Other Options are Incorrect:** * **Options A & D (Mannitol):** While mannitol is used to measure GFR (similar to inulin), it does not measure RPF. * **Option C (Creatinine):** Creatinine is used to estimate GFR in clinical practice, but it is slightly secreted by tubules, making it less accurate than inulin. Using two GFR markers (Inulin and Creatinine) cannot determine the FF as the RPF component is missing. **High-Yield Facts for NEET-PG:** * **Normal FF:** Approximately **20%** (0.20). * **Clinical Significance:** FF increases in conditions like **Congestive Heart Failure** and **Renal Artery Stenosis** (due to a disproportionate decrease in RPF compared to GFR, maintained by Angiotensin II-mediated efferent vasoconstriction). * **Inulin Clearance = GFR; PAH Clearance = ERPF.** * **Creatinine Clearance** slightly overestimates GFR (due to tubular secretion).

Q: About ADH secretion all are true except?

Level increased by low plasma osmolality.. **Explanation:** The correct answer is **D** because it is a false statement. Antidiuretic Hormone (ADH), also known as Vasopressin, is primarily regulated by plasma osmolality. Secretion is **stimulated by high plasma osmolality** (detected by osmoreceptors in the hypothalamus) and inhibited by low plasma osmolality. When osmolality is low (dilute blood), ADH levels drop to allow the excretion of excess water. **Analysis of other options:** * **Option A:** ADH secretion increases during the **early postoperative period** as part of the body's surgical stress response. Factors like pain, anxiety, and blood loss (hypovolemia) are potent non-osmotic stimuli for ADH release. * **Option B:** ADH acts on the V2 receptors in the **Distal Convoluted Tubule (DCT)** and collecting ducts. It facilitates the insertion of Aquaporin-2 channels, thereby increasing water permeability and reabsorption. * **Option C:** ADH is a classic example of **neurosecretion**. It is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and transported axonally to the posterior pituitary (neurohypophysis) for storage and release. **High-Yield Clinical Pearls for NEET-PG:** * **Most sensitive stimulus:** A change in plasma osmolality as small as 1%. * **Most potent stimulus:** Severe hypovolemia/hypotension (though it requires a >10% change in volume). * **SIADH:** Characterized by inappropriately high ADH despite low plasma osmolality, leading to dilutional hyponatremia. * **Diabetes Insipidus:** Central (lack of ADH) or Nephrogenic (resistance to ADH), resulting in polyuria and high plasma osmolality.

Q: Which substance undergoes both filtration and reabsorption in the nephron?

Glucose. **Explanation:** The correct answer is **Glucose**. In a healthy individual, glucose is freely filtered at the glomerulus because of its small molecular size. Subsequently, it undergoes **100% reabsorption** in the **Proximal Convoluted Tubule (PCT)** via the SGLT-2 (Sodium-Glucose Linked Cotransporter) and SGLT-1 transporters. Under normal physiological conditions, no glucose is excreted in the urine. **Analysis of Incorrect Options:** * **Inulin (Option A):** Inulin is the "gold standard" for measuring GFR because it is **freely filtered** but is **neither reabsorbed nor secreted** by the renal tubules. (Note: The option says "Insulin," which is a hormone; however, in renal physiology contexts, "Inulin" is the classic distractor). * **Creatinine (Option C):** Creatinine is freely filtered and is **not reabsorbed**. It undergoes a small amount of tubular secretion, which is why creatinine clearance slightly overestimates the actual GFR. * **Para-aminohippuric acid (PAH) (Option D):** PAH is freely filtered and **extensively secreted** by the PCT. It is used to measure **Renal Plasma Flow (RPF)** because it is almost completely cleared from the blood in a single pass through the kidney. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Threshold for Glucose:** Glucose starts appearing in the urine (glycosuria) when blood glucose levels exceed **180 mg/dL**. * **Transport Maximum ($T_m$):** The $T_m$ for glucose is approximately **375 mg/min** in men and **300 mg/min** in women. * **SGLT-2 Inhibitors:** Drugs like Dapagliflozin inhibit glucose reabsorption in the PCT and are used in managing Diabetes Mellitus.

Q: All of the following cause renal vasoconstriction except?

Carbon monoxide. **Explanation:** The regulation of renal blood flow (RBF) and glomerular filtration rate (GFR) depends on the balance between vasoconstrictors and vasodilators. **Why Carbon Monoxide (CO) is correct:** Carbon monoxide acts as a potent **vasodilator** in the renal vasculature. It functions similarly to Nitric Oxide (NO) by stimulating the enzyme **soluble guanylyl cyclase**, which increases intracellular **cGMP** levels. This leads to smooth muscle relaxation and increased renal blood flow. Additionally, CO inhibits the release of endothelin, further promoting vasodilation. **Why the other options are incorrect:** * **Angiotensin II (Option A):** A powerful vasoconstrictor that preferentially constricts the **efferent arterioles** to maintain GFR when renal perfusion pressure is low. * **Norepinephrine (Option B):** Released via sympathetic stimulation, it acts on **$\alpha_1$ adrenoceptors** to cause significant constriction of both afferent and efferent arterioles, reducing RBF. * **Endothelin (Option D):** One of the most potent endogenous vasoconstrictors produced by damaged or stressed endothelial cells. It plays a role in pathological states like acute renal failure. **High-Yield Clinical Pearls for NEET-PG:** * **Vasodilators of the Kidney:** Prostaglandins ($PGE_2$, $PGI_2$), Nitric Oxide (NO), Bradykinin, Atrial Natriuretic Peptide (ANP), Dopamine (at low doses), and Carbon Monoxide. * **Vasoconstrictors of the Kidney:** Angiotensin II, Norepinephrine, Epinephrine, Endothelin, and ADH (Vasopressin). * **Prostaglandin Role:** They act as protective vasodilators during states of high vasoconstriction. This is why **NSAIDs** (which inhibit prostaglandins) can cause acute kidney injury by allowing unopposed vasoconstriction.

Q: What is true about the H+ secretion by the proximal convoluted tubule (PCT)?

It is meant for reabsorption of the filtered bicarbonate.. In the Proximal Convoluted Tubule (PCT), H+ secretion is primarily coupled with the reabsorption of filtered bicarbonate ($HCO_3^-$), a process essential for maintaining acid-base balance. ### Why Option C is Correct: Approximately 85-90% of filtered bicarbonate is reabsorbed in the PCT. This occurs via the **$Na^+$-$H^+$ Exchanger (NHE3)** on the apical membrane, which secretes $H^+$ into the lumen. In the lumen, $H^+$ combines with filtered $HCO_3^-$ to form $H_2CO_3$. **Carbonic Anhydrase (Type IV)** on the brush border breaks this into $CO_2$ and $H_2O$, which diffuse into the cell. Inside, **Carbonic Anhydrase (Type II)** reforms $HCO_3^-$, which is then transported into the blood. Thus, $H^+$ secretion here is a "shuttle" for bicarbonate reabsorption rather than a means to excrete acid. ### Why Other Options are Incorrect: * **Option A:** Acidification of urine (lowering pH to ~4.5) occurs in the **Alpha-Intercalated cells** of the distal tubule and collecting duct via $H^+$-ATPase. The PCT does not significantly lower urine pH. * **Option B:** $H^+$ secretion in the PCT is **not** against a steep concentration gradient. The luminal pH in the PCT remains relatively high (~6.7-7.0). Active transport against a steep gradient occurs only in the distal nephron. * **Option D:** Diffusion trapping (ammoniagenesis) primarily occurs in the **collecting ducts**, where $NH_3$ combines with $H^+$ to form $NH_4^+$, which is "trapped" and excreted. ### High-Yield Facts for NEET-PG: * **Acetazolamide** (Carbonic Anhydrase Inhibitor) acts on the PCT, blocking this $H^+$ secretion and leading to bicarbonate loss (Proximal Renal Tubular Acidosis/Type 2 RTA). * **NHE3** is the primary transporter for $H^+$ secretion in the PCT. * The PCT is responsible for the **bulk reabsorption** of $HCO_3^-$, while the distal nephron is responsible for **net acid excretion** and the generation of "new" bicarbonate.

Q: Maximum phosphate is reabsorbed in which part of the nephron?

Proximal Convoluted Tubule (PCT). **Explanation:** The **Proximal Convoluted Tubule (PCT)** is the primary site for the reabsorption of the majority of solutes filtered by the glomerulus. Approximately **80% of filtered phosphate** is reabsorbed in the PCT. **Why PCT is the correct answer:** Phosphate reabsorption in the PCT occurs via **Sodium-Phosphate cotransporters (NaPi-IIa and NaPi-IIc)** located on the apical membrane. This process is secondary active transport, driven by the sodium gradient established by the Na+/K+ ATPase pump. The PCT is the only segment where significant, regulated phosphate transport occurs, making it the most critical site for phosphate homeostasis. **Why other options are incorrect:** * **Distal Convoluted Tubule (DCT) & Collecting Duct:** Only a negligible amount of phosphate (approx. 5-10%) is reabsorbed in the distal segments. These areas are more focused on fine-tuning electrolytes like sodium, potassium, and calcium under hormonal influence (e.g., Aldosterone). * **Loop of Henle:** This segment is primarily involved in the concentration of urine and the reabsorption of sodium, chloride, and water (Thin Descending and Thick Ascending Limbs), but it plays no significant role in phosphate reabsorption. **High-Yield Clinical Pearls for NEET-PG:** 1. **Parathyroid Hormone (PTH):** PTH is the most important regulator of phosphate. It **inhibits** phosphate reabsorption in the PCT by causing the internalization of NaPi-IIa transporters, leading to **phosphaturia**. 2. **FGF-23:** A phosphatonin that also inhibits PCT phosphate reabsorption. 3. **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of phosphate (hypophosphatemia), glucose, amino acids, and bicarbonate in the urine. 4. **Threshold:** Phosphate is a "threshold substance"; once the plasma concentration exceeds the transport maximum (TmP), the excess is rapidly excreted in the urine.

Question 1

What is the primary function of Antidiuretic Hormone (ADH)?

Accepted Answer

Promotes water reabsorption

Answer

Inhibits water excretion

Answer

Facilitates sodium absorption

Answer

Stimulates potassium secretion

Question 2

Ethanol increases urine output by which of the following mechanisms?

Accepted Answer

Suppressing ADH secretion

Answer

Inhibiting Na+ reabsorption

Answer

Increasing glomerular filtration rate

Answer

Suppressing H+ secretion

Question 3

What is the most potent stimulus for renin release?

Accepted Answer

Hypotension

Answer

Sympathetic stimulation

Answer

Decreased sodium chloride in the distal convoluted tubule

Answer

Prostacyclin

Question 4

Filtration fraction can be best determined by which pair of substances?

Accepted Answer

PAH and Inulin

Answer

Inulin and mannitol

Answer

Inulin and creatinine

Answer

PAH and mannitol

Question 5

About ADH secretion all are true except?

Accepted Answer

Level increased by low plasma osmolality.

Answer

Secretion increased in early postoperative period.

Answer

Causes increased permeability of DCT.

Answer

Neurosecretion.

Question 6

Which substance undergoes both filtration and reabsorption in the nephron?

Accepted Answer

Glucose

Answer

Insulin

Answer

Creatinine

Answer

Para-aminohippuric acid

Question 7

All of the following cause renal vasoconstriction except?

Accepted Answer

Carbon monoxide

Answer

Angiotensin II

Answer

Norepinephrine

Answer

Endothelin

Question 8

What is true about the H+ secretion by the proximal convoluted tubule (PCT)?

Accepted Answer

It is meant for reabsorption of the filtered bicarbonate.

Answer

It is for acidification of urine.

Answer

It occurs against the concentration gradient for H+.

Answer

It is an example of diffusion trapping.

Question 9

Maximum phosphate is reabsorbed in which part of the nephron?

Accepted Answer

Proximal Convoluted Tubule (PCT)

Answer

Distal Convoluted Tubule (DCT)

Answer

Loop of Henle

Answer

Collecting Duct

Question 10

Renin secretion is increased by all of the following except:

Accepted Answer

Sympathetic nerve stimulation

Answer

Sodium load in the proximal convoluted tubule (PCT)

Answer

Low afferent arteriolar pressure

Answer

Sodium load in the distal convoluted tubule (DCT)

Renal Physiology — MCQs

Renal Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?