Renal Physiology Practice Questions

Q: What is the cause of polyuria?

Hyperglycemia. **Explanation:** The correct answer is **Hyperglycemia**. **Mechanism of Polyuria in Hyperglycemia:** Polyuria in hyperglycemia occurs due to **osmotic diuresis** [3]. When blood glucose levels exceed the renal threshold (approximately 180 mg/dL), the proximal convoluted tubules cannot reabsorb the excess glucose. This glucose remains in the renal tubules, acting as an osmotically active particle that holds water within the lumen, preventing its reabsorption [2]. This results in an increased volume of urine output. **Analysis of Incorrect Options:** * **B. Hypoglycemia:** Low blood sugar does not affect renal osmotic pressure and typically presents with autonomic symptoms (sweating, tremors) rather than polyuria. * **C. Decreased fluid intake:** This leads to dehydration, triggering the release of Antidiuretic Hormone (ADH) [1], which causes the kidneys to conserve water, resulting in **oliguria** (decreased urine output) [4], not polyuria. * **D. Hypocalcemia:** While **Hypercalcemia** is a known cause of polyuria (by inducing nephrogenic diabetes insipidus and interfering with ADH action), hypocalcemia typically causes neuromuscular irritability (tetany) and does not cause polyuria. **NEET-PG High-Yield Pearls:** * **Common causes of Polyuria:** Diabetes Mellitus (osmotic diuresis), Diabetes Insipidus (deficiency or resistance to ADH), Hypercalcemia, and Hypokalemia. * **Renal Threshold for Glucose:** ~180 mg/dL. * **Definition of Polyuria:** Urine output >3 L/day in adults. * **Differential Diagnosis:** Always differentiate between water diuresis (low urine osmolality, e.g., DI) and solute diuresis (high urine osmolality, e.g., DM) [3].

Q: What is the primary mechanism by which water is reabsorbed from the proximal convoluted tubule?

Osmosis. ### Explanation **Correct Option: D (Osmosis)** In the Proximal Convoluted Tubule (PCT), approximately 65% of filtered water is reabsorbed. This process is driven by **osmosis**. As solutes (primarily Sodium via the Na+/K+ ATPase pump, along with glucose and amino acids) are actively transported from the tubular lumen into the peritubular capillaries, an osmotic gradient is created. Water follows these solutes "passively" to maintain osmotic equilibrium. This is termed **obligatory water reabsorption** because it occurs regardless of the body's hydration status or ADH levels. **Why other options are incorrect:** * **A. Active Transport:** Water movement is never directly powered by ATP. While it depends on the active transport of *solutes*, the movement of water molecules themselves is a physical response to concentration gradients. * **B. Passive Transport:** While osmosis is a form of passive transport, "Osmosis" is the more specific and accurate physiological term for the movement of solvent (water) across a semi-permeable membrane. * **C. Facilitated Diffusion:** This involves the use of carrier proteins to move solutes down a concentration gradient. While water moves through channels called **Aquaporin-1** in the PCT, the driving force is the osmotic pressure, not a carrier-mediated diffusion process. **High-Yield Clinical Pearls for NEET-PG:** * **Isotonic Reabsorption:** The fluid leaving the PCT remains **isotonic** to plasma (300 mOsm/L) because water and solutes are reabsorbed in equal proportions. * **Aquaporin-1:** These are the specific water channels located in the PCT and the descending limb of the Loop of Henle. * **Solvent Drag:** As water moves via osmosis through paracellular pathways, it "drags" other solutes like K+, Ca2+, and Mg2+ along with it.

Q: Inulin clearance is measured to estimate which of the following?

Glomerular filtration rate (GFR). **Explanation:** **1. Why Glomerular Filtration Rate (GFR) is correct:** Inulin is a fructose polymer that serves as the **gold standard** for measuring GFR because it possesses specific physiological properties: it is freely filtered at the glomerulus, but it is **neither reabsorbed nor secreted** by the renal tubules. Furthermore, it is not metabolized or synthesized by the kidney. Therefore, the amount of inulin filtered per unit time equals the amount excreted in the urine ($U_{in} \times V = P_{in} \times GFR$), making its clearance exactly equal to the GFR. **2. Why the other options are incorrect:** * **Extracellular Fluid (ECF):** While inulin distributes into the ECF, its *clearance* (rate of removal from plasma) measures GFR. To measure ECF volume, substances like **Inulin, Mannitol, or Sucrose** are used via the dilution method, but this is a volume measurement, not a clearance measurement. * **Intracellular Fluid (ICF):** ICF cannot be measured directly. It is calculated by subtracting ECF from Total Body Water (TBW). * **Plasma Volume:** This is measured using substances that bind to albumin (e.g., **Evans Blue dye**) or radioactive **Albumin ($^{125}$I-RISA)**, as they remain confined to the vascular compartment. **3. High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Clearance:** Used clinically to estimate GFR. It slightly **overestimates** GFR because a small amount of creatinine is secreted by the tubules. * **Para-aminohippuric acid (PAH) Clearance:** Used to measure **Effective Renal Plasma Flow (ERPF)** because it is both filtered and almost completely secreted. * **Formula:** $Clearance (C) = \frac{U \times V}{P}$ (where $U$ = Urine concentration, $V$ = Urine flow rate, $P$ = Plasma concentration). * If Clearance Inulin Clearance $\rightarrow$ Net Secretion.

Q: All of the following are actions of Atrial Natriuretic Peptide except:

Mesangial constriction. **Explanation:** Atrial Natriuretic Peptide (ANP) is a hormone secreted by the cardiac atria in response to stretch (volume overload). Its primary goal is to lower blood pressure and volume by promoting natriuresis (sodium excretion) and diuresis. **Why "Mesangial constriction" is the correct answer:** ANP actually causes **mesangial cell relaxation**, not constriction. By relaxing these cells, ANP increases the effective surface area of the glomerular capillaries available for filtration, which significantly increases the Glomerular Filtration Rate (GFR). **Analysis of other options:** * **Afferent arteriole dilation:** ANP increases GFR by simultaneously dilating the afferent arteriole and constricting the efferent arteriole. This "push-pull" mechanism increases glomerular hydrostatic pressure. * **Decreased sodium absorption in the PCT:** ANP inhibits the secretion of Renin and Angiotensin II. Since Angiotensin II normally stimulates sodium reabsorption in the proximal convoluted tubule (PCT), ANP indirectly decreases sodium reabsorption here. * **Inhibition of sodium reabsorption in the medullary collecting duct:** This is a major direct action of ANP. It acts on the inner medullary collecting duct to inhibit sodium channels (ENaC), leading to increased sodium excretion. **High-Yield NEET-PG Pearls:** * **Mechanism of Action:** ANP acts via **membrane-bound Guanylyl Cyclase**, increasing intracellular **cGMP** (similar to Nitric Oxide). * **Antagonist to RAAS:** ANP is the physiological antagonist to the Renin-Angiotensin-Aldosterone System. * **Systemic Effect:** It causes systemic vasodilation, leading to decreased peripheral resistance and blood pressure. * **BNP (Brain Natriuretic Peptide):** Secreted by ventricles; used clinically as a marker for heart failure.

Q: A high plasma renin activity (PRA) is expected in patients?

Taking an angiotensin-converting enzyme inhibitor. **Explanation:** The correct answer is **C. Taking an angiotensin-converting enzyme inhibitor.** **Mechanism of Action:** Renin secretion is primarily regulated by a **negative feedback loop** involving Angiotensin II (AT-II). Under normal physiological conditions, AT-II acts on the juxtaglomerular (JG) cells to inhibit further renin release. ACE inhibitors block the conversion of Angiotensin I to Angiotensin II. The resulting decrease in AT-II levels removes this "negative feedback brake," leading to a compensatory and significant increase in **Plasma Renin Activity (PRA)**. **Analysis of Incorrect Options:** * **A. Sodium Overload:** Increased sodium delivery to the macula densa and increased ECF volume suppress renin release. Renin is stimulated by sodium *depletion* or hypotension. * **B. Beta Blockers:** JG cells possess **$\beta_1$ receptors** that stimulate renin release via the sympathetic nervous system. Beta-blockers inhibit these receptors, thereby decreasing PRA. * **D. Conn Syndrome (Primary Hyperaldosteronism):** This condition involves autonomous aldosterone secretion (usually from an adrenal adenoma). The resulting hypertension and sodium retention cause a feedback suppression of the renin-angiotensin system, leading to **low PRA**. **NEET-PG High-Yield Pearls:** * **PRA vs. PRC:** Plasma Renin Activity (PRA) measures the ability of renin to generate Angiotensin I, whereas Plasma Renin Concentration (PRC) measures the actual amount of the enzyme. Both are elevated with ACE inhibitors and ARBs. * **Screening for Conn’s:** The **Aldosterone-to-Renin Ratio (ARR)** is the screening test of choice. A high ratio (High Aldosterone + Low Renin) is diagnostic. * **Renin Stimulants:** Think of the "3 Ds": **D**iuretics, **D**ehydration, and **D**ecreased BP (Hypotension). All increase PRA.

Q: What is the cause of atonic bladder?

Injury to the sacral plexus. **Explanation:** The micturition reflex is primarily mediated by the **parasympathetic nervous system** via the **pelvic nerves**, which originate from the **sacral segments (S2–S4)** of the spinal cord. **Why Option A is Correct:** An **atonic bladder** (or non-reflexive bladder) occurs when the sensory (afferent) or motor (efferent) nerve fibers connecting the bladder to the spinal cord are destroyed. Injury to the **sacral plexus** or the sacral cord segments interrupts the parasympathetic supply. This results in the loss of detrusor muscle tone and the micturition reflex. The bladder becomes thin-walled and greatly distended, leading to **overflow incontinence**, where urine dribbles out only when the bladder is physically full. **Why Other Options are Incorrect:** * **B. Injury to the upper thoracic cord:** This results in a **spastic (automatic) bladder**. Initially, there is spinal shock (temporary atony), but once the reflex arc below the injury recovers, the bladder empties automatically when filled, as the inhibitory control from the brain is lost. * **C. Pregnancy:** Pregnancy typically causes increased frequency of micturition due to hormonal changes and mechanical pressure on the bladder, but it does not cause an atonic state. * **D. UTI:** Infections cause bladder irritability and "urgency," leading to a hyperactive state rather than atony. **High-Yield Clinical Pearls for NEET-PG:** * **Tabes Dorsalis:** A classic cause of atonic bladder due to syphilis destroying the dorsal (sensory) root fibers. * **Atonic Bladder:** Characterized by high residual volume and overflow incontinence. * **Spastic Bladder:** Occurs in lesions above the sacral centers (e.g., cervical or thoracic cord injuries). * **Nerve Supply:** Remember **"S2, 3, 4 keeps the pee off the floor"** (Parasympathetic supply for voiding).

Question 1

All of the following statements about renal regulation in a patient with hypovolemic shock are true, except:

Accepted Answer

Constriction of afferent arterioles more than the efferent arteriole

Answer

Renal vasoconstriction

Answer

Reduced renal plasma flow

Answer

Reduced glomerular filtration rate

Question 2

What is the cause of polyuria?

Accepted Answer

Hyperglycemia

Answer

Hypoglycemia

Answer

Decreased fluid intake

Answer

Hypocalcemia

Question 3

What is the primary mechanism by which water is reabsorbed from the proximal convoluted tubule?

Accepted Answer

Osmosis

Answer

Active transport

Answer

Passive transport

Answer

Facilitated diffusion

Question 4

Inulin clearance is measured to estimate which of the following?

Accepted Answer

Glomerular filtration rate (GFR)

Answer

Extracellular fluid (ECF)

Answer

Intracellular fluid (ICF)

Answer

Plasma volume

Question 5

In the presence of vasopressin, which part of the nephron becomes more permeable to water?

Accepted Answer

Cortical collecting duct

Answer

Proximal tubule

Answer

Loop of Henle

Answer

Distal tubule

Question 6

All of the following are actions of Atrial Natriuretic Peptide except:

Accepted Answer

Mesangial constriction

Answer

Afferent arteriole dilation

Answer

Decreased sodium absorption in the proximal convoluted tubule

Answer

Inhibition of sodium reabsorption in the medullary collecting duct

Question 7

A high plasma renin activity (PRA) is expected in patients?

Accepted Answer

Taking an angiotensin-converting enzyme inhibitor

Answer

With sodium overload

Answer

Presently taking a beta blocker

Answer

With Conn syndrome

Question 8

What is the cause of atonic bladder?

Accepted Answer

Injury to the sacral plexus

Answer

Injury to the upper thoracic cord

Answer

Pregnancy

Answer

Urinary tract infection (UTI)

Question 9

What is the primary ion responsible for the hyperosmolarity of the renal medulla?

Accepted Answer

Chloride (Cl-)

Answer

Potassium (K+)

Answer

Sodium (Na+)

Answer

Glucose

Question 10

Potassium reabsorption in the kidney occurs primarily in which segment(s)?

Accepted Answer

None of the above statements accurately describe potassium reabsorption

Answer

Proximal Convoluted Tubule (PCT) and Distal Convoluted Tubule (DCT)

Answer

Proximal Convoluted Tubule (PCT) and Collecting Duct (CD)

Answer

Coupled with sodium ions in the loop of Henle

Renal Physiology — MCQs

Renal Physiology — MCQs

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