Renal Physiology Practice Questions

Q: GFR decreases with the following:

Hypotension. **Explanation:** The Glomerular Filtration Rate (GFR) is determined by the **Starling forces** acting across the glomerular capillaries. The primary driving force for filtration is the **Glomerular Capillary Hydrostatic Pressure ($P_{GC}$)**, which is directly influenced by the systemic arterial blood pressure. **1. Why Hypotension is Correct:** Hypotension (a decrease in systemic blood pressure) leads to a reduction in renal perfusion pressure. This directly lowers the $P_{GC}$. According to the GFR formula: $GFR = K_f \times [(P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})]$, a drop in $P_{GC}$ reduces the net filtration pressure, thereby decreasing the GFR. While autoregulation (myogenic mechanism and tubuloglomerular feedback) attempts to maintain GFR, severe or persistent hypotension overrides these mechanisms, leading to pre-renal azotemia. **2. Why the other options are incorrect:** * **Hypoproteinemia:** This condition involves a decrease in plasma proteins (e.g., albumin), which lowers the **Glomerular Capillary Oncotic Pressure ($\pi_{GC}$)**. Since oncotic pressure opposes filtration, a decrease in $\pi_{GC}$ actually **increases** the GFR. * **Hypertension:** High systemic blood pressure increases the $P_{GC}$. While renal autoregulation works to keep GFR stable by constricting the afferent arteriole, the physiological tendency of hypertension is to **increase** or maintain GFR, not decrease it. **Clinical Pearls for NEET-PG:** * **Afferent Arteriole:** Constriction decreases GFR; Dilation increases GFR. * **Efferent Arteriole:** Constriction (at moderate levels) increases GFR by backing up pressure; however, severe constriction can decrease GFR due to reduced renal blood flow. * **Starling Forces:** $P_{GC}$ is the only force that significantly favors filtration; $\pi_{GC}$ and $P_{BS}$ (Bowman’s space hydrostatic pressure) are the primary forces opposing it.

Q: Which of the following substances is primarily filtered by the glomerulus with minimal secretion or reabsorption in the renal tubules?

Inulin. ### Explanation **Correct Answer: B. Inulin** Inulin is a fructose polymer that serves as the **gold standard** for measuring the Glomerular Filtration Rate (GFR). Its unique physiological properties make it ideal for this purpose: it is freely filtered at the glomerulus and is neither reabsorbed nor secreted by the renal tubules. Consequently, the amount of inulin filtered is exactly equal to the amount excreted in the urine ($Filtered\ Load = Excretion\ Rate$). **Analysis of Incorrect Options:** * **A. Sodium:** Sodium is freely filtered but undergoes massive reabsorption (approximately 99%) throughout the nephron, primarily in the proximal convoluted tubule (PCT) and the Loop of Henle. * **C. Glucose:** Under normal physiological conditions, glucose is freely filtered but **100% reabsorbed** in the PCT via SGLT-2 and SGLT-1 transporters. It only appears in urine if the blood glucose level exceeds the renal threshold (approx. 180 mg/dL). * **D. Amino acids:** Similar to glucose, amino acids are freely filtered but are almost entirely reabsorbed in the PCT to conserve essential nutrients. **High-Yield Clinical Pearls for NEET-PG:** * **Creatinine vs. Inulin:** While Inulin is the gold standard, **Creatinine** is used clinically to estimate GFR. Note that Creatinine slightly *overestimates* GFR because a small amount is secreted by the tubules. * **Para-aminohippuric acid (PAH):** Unlike inulin, PAH is both filtered and aggressively secreted. It is used to measure **Renal Plasma Flow (RPF)** because it is almost completely cleared from the blood in a single pass. * **Criteria for GFR Marker:** Must be non-toxic, not metabolized by the kidney, and physiologically inert.

Q: Normal filtration fraction is around:

0.2. **Explanation:** **Filtration Fraction (FF)** is the ratio of the Glomerular Filtration Rate (GFR) to the Renal Plasma Flow (RPF). It represents the fraction of blood plasma that is actually filtered across the glomerular capillaries into the Bowman’s space. The formula is: **FF = GFR / RPF** * **Normal GFR:** ~125 mL/min * **Normal RPF:** ~625 mL/min * **Calculation:** 125 / 625 = **0.2 (or 20%)** **Why Option C is correct:** In a healthy adult, approximately 20% of the plasma entering the kidneys is filtered. This value (0.2) is a physiological constant under normal conditions, ensuring adequate clearance of waste while maintaining sufficient peritubular capillary pressure for reabsorption. **Why other options are incorrect:** * **Option A (0.05) and B (0.1):** These values are too low. A filtration fraction this low would indicate either a significant drop in GFR (e.g., acute kidney injury) or an excessive increase in renal plasma flow, leading to insufficient waste excretion. **High-Yield NEET-PG Clinical Pearls:** 1. **Effect of Efferent Arteriole Constriction:** This increases FF. Constriction increases glomerular hydrostatic pressure (increasing GFR) but decreases RPF; since GFR increases more than RPF decreases, the ratio rises. 2. **Hypovolemia/Hemorrhage:** In states of low blood volume, the body maintains GFR via Angiotensin II (which constricts the efferent arteriole). This leads to a **rise in FF** to compensate for decreased renal blood flow. 3. **Peritubular Capillaries:** A high FF increases the oncotic pressure in the peritubular capillaries, which serves as a primary driving force for tubular reabsorption.

Q: Free water clearance by the kidney is increased by which of the following?

Diabetes insipidus. ### Explanation **Concept of Free Water Clearance ($C_{H_2O}$):** Free water clearance represents the volume of solute-free water excreted by the kidneys per unit of time. It is calculated as the difference between total urine flow ($V$) and osmolar clearance ($C_{osm}$): $C_{H_2O} = V - C_{osm}$. * **Positive $C_{H_2O}$:** The kidney is excreting dilute urine (excess water). * **Negative $C_{H_2O}$:** The kidney is concentrating urine (conserving water). **Why Diabetes Insipidus (DI) is Correct:** In DI (whether Central or Nephrogenic), there is either a lack of ADH or a lack of renal response to it. Without ADH, the collecting ducts remain impermeable to water. Large volumes of hypotonic, dilute urine are excreted. Since $V$ is very high and $U_{osm}$ is very low, the **Free Water Clearance increases significantly.** **Why Other Options are Incorrect:** * **Renal Failure:** In chronic kidney disease, the kidney loses its ability to both concentrate and dilute urine (isosthenuria). The $C_{H_2O}$ tends to approach zero. * **Diuretic Therapy:** Most diuretics (like Loop diuretics) interfere with the medullary osmotic gradient. While they increase urine volume, they also increase solute excretion ($C_{osm}$). This typically results in a $C_{H_2O}$ that moves toward zero. * **Diabetes Mellitus:** This causes **osmotic diuresis** due to glucose in the tubules. While urine volume increases, the solute load ($C_{osm}$) is very high, which actually decreases the free water clearance. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Dilution:** The "diluting segments" of the nephron are the Thick Ascending Limb (TAL) and the Distal Convoluted Tubule (DCT). * **ADH Action:** ADH acts on **V2 receptors** in the late distal tubule and collecting ducts to insert **Aquaporin-2** channels. * **Formula:** $C_{H_2O} = V \times (1 - U_{osm}/P_{osm})$. If $U_{osm} < P_{osm}$, $C_{H_2O}$ is positive.

Q: Which of the following is not a site of renal tubular secretion?

Collecting tubule. **Explanation:** Renal tubular secretion is the process by which substances are moved from the peritubular capillaries into the tubular lumen to be excreted in the urine. While most segments of the nephron participate in this process, the **Collecting Tubule** (specifically the cortical and medullary collecting ducts) is primarily involved in the **reabsorption** of water (via ADH) and urea, and the **fine-tuning** of electrolyte balance, rather than being a primary site for the active secretion of metabolic wastes or foreign substances. * **Proximal Convoluted Tubule (PCT):** This is the most active site for secretion. It secretes organic acids and bases, including bile salts, oxalate, urate, and drugs like penicillin and salicylates. It also secretes $H^+$ ions. * **Distal Convoluted Tubule (DCT):** This segment is crucial for the secretion of $K^+$ and $H^+$ ions, primarily under the influence of aldosterone, to maintain acid-base and electrolyte balance. * **Loop of Henle:** While primarily known for the countercurrent multiplier system, the thin descending limb is involved in the secretion of urea into the tubular fluid (urea recycling). **NEET-PG High-Yield Pearls:** 1. **PCT** is the site for the secretion of **Para-aminohippuric acid (PAH)**, which is used to measure Renal Plasma Flow (RPF). 2. **Potassium ($K^+$)** is unique because it is both reabsorbed (PCT/Loop) and secreted (DCT/Collecting duct). The net urinary excretion of $K^+$ is determined mainly by secretion in the late distal tubule and cortical collecting duct. 3. **Ammonia ($NH_3$)** is synthesized and secreted primarily in the **PCT**.

Q: Tamm-Horsfall protein is secreted by epithelial cells of which part of the nephron?

Loop of Henle. **Explanation:** **Tamm-Horsfall protein (THP)**, also known as **Uromodulin**, is the most abundant protein excreted in normal urine. It is a high-molecular-weight glycoprotein synthesized and secreted exclusively by the epithelial cells of the **thick ascending limb (TAL) of the Loop of Henle**. **Why Option B is Correct:** The TAL cells produce THP and anchor it to the luminal membrane. Its primary functions include preventing calcium oxalate stone formation, providing defense against urinary tract infections (UTIs) by binding to *E. coli* fimbriae, and regulating salt transport. Because it is secreted specifically in the TAL, it serves as a specific marker for this segment of the nephron. **Why Other Options are Incorrect:** * **A. Proximal Convoluted Tubule:** This segment is primarily involved in the reabsorption of filtered proteins (like albumin) rather than the secretion of specific structural glycoproteins like THP. * **C. Distal Convoluted Tubule:** While some older texts suggest minimal secretion in the early DCT, the definitive and primary site recognized in high-yield physiology is the TAL of the Loop of Henle. * **D. Glomerulus:** The glomerulus acts as a filtration barrier. THP is not filtered from the blood; it is added to the tubular fluid after the filtrate has passed the glomerulus. **Clinical Pearls for NEET-PG:** 1. **Cast Formation:** THP forms the **matrix of all urinary casts**. In conditions of low flow or high acidity, it gels to trap cells (e.g., RBC casts, WBC casts). 2. **Hyaline Casts:** These are composed almost entirely of Tamm-Horsfall protein. 3. **Myeloma Kidney:** Bence-Jones proteins can precipitate with THP, leading to intratubular obstruction and renal failure. 4. **Genetic Link:** Mutations in the Uromodulin gene are associated with Medullary Cystic Kidney Disease.

Q: Which of the following substances is reabsorbed along with sodium in the early portion of the proximal tubule?

All of the above. **Explanation:** In the **early proximal convoluted tubule (PCT)**, the reabsorption of sodium is coupled with several essential solutes through specific secondary active transport mechanisms. This process is driven by the sodium-potassium ATPase pump on the basolateral membrane, which creates a low intracellular sodium concentration and a negative membrane potential. 1. **Glucose:** It is reabsorbed via **SGLT-2** (Sodium-Glucose Co-transporter 2) in the early PCT. This is a symport mechanism where glucose moves against its gradient along with sodium. 2. **Amino Acids:** Various sodium-dependent amino acid transporters (symporters) facilitate the reabsorption of almost 100% of filtered amino acids in this segment. 3. **Bicarbonate ($HCO_3^-$):** Sodium reabsorption is coupled with hydrogen ion secretion via the **NHE3** (Sodium-Hydrogen Exchanger). The secreted $H^+$ reacts with filtered $HCO_3^-$ to form $CO_2$ and $H_2O$ (catalyzed by Carbonic Anhydrase), which then enters the cell. Thus, $HCO_3^-$ reabsorption is indirectly but fundamentally linked to sodium transport in the early PCT. **Why "All of the above" is correct:** The early PCT is the primary site for "bulk reabsorption." While the late PCT focuses more on sodium chloride (NaCl) reabsorption, the early portion is specialized for the co-transport of sodium with organic solutes (glucose, amino acids) and bicarbonate. **High-Yield Clinical Pearls for NEET-PG:** * **Fanconi Syndrome:** A generalized dysfunction of the PCT leading to the loss of glucose, amino acids, bicarbonate, and phosphates in the urine. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** These drugs act specifically in the early PCT to treat Diabetes Mellitus by promoting glucosuria. * **Carbonic Anhydrase Inhibitors (Acetazolamide):** Act in the PCT to inhibit $HCO_3^-$ reabsorption, leading to alkaline urine and metabolic acidosis.

Q: Constriction of efferent arteriole produces which of the following effects?

All of the above. **Explanation:** The constriction of the efferent arteriole is a key physiological mechanism for regulating the Glomerular Filtration Rate (GFR) and tubular reabsorption. 1. **Biphasic response on GFR:** At low to moderate levels of constriction, GFR increases because the outflow resistance raises the glomerular hydrostatic pressure ($P_{GC}$). However, at high levels of constriction, the GFR actually decreases. This is because severe constriction significantly reduces Renal Plasma Flow (RPF), causing the filtration fraction to rise so sharply that the plasma oncotic pressure ($π_{GC}$) increases rapidly along the capillary, eventually opposing filtration. 2. **Increased peritubular oncotic pressure:** As blood passes through the glomerulus, fluid is filtered out, leaving behind concentrated plasma proteins. Efferent constriction increases the filtration fraction, meaning a higher proportion of plasma is filtered. Consequently, the blood entering the peritubular capillaries has a significantly higher colloid osmotic (oncotic) pressure. 3. **Increased peritubular absorption:** Tubular reabsorption is governed by Starling forces in the peritubular capillaries. The increased oncotic pressure (noted above) combined with a *decreased* peritubular hydrostatic pressure (due to the resistance at the efferent arteriole) creates a powerful pressure gradient that favors the movement of fluid from the interstitium back into the capillaries. **High-Yield Facts for NEET-PG:** * **Angiotensin II** preferentially constricts the efferent arteriole to maintain GFR during states of low renal perfusion (e.g., dehydration). * **ACE Inhibitors** block this mechanism, leading to efferent vasodilation, which can cause a precipitous drop in GFR in patients with renal artery stenosis. * **Filtration Fraction (FF) = GFR / RPF.** Efferent constriction always increases FF because it decreases RPF more than it affects GFR.

Q: ANP acts at which part of the nephron?

Collecting tubule. **Explanation:** Atrial Natriuretic Peptide (ANP) is a potent hormone secreted by the cardiac atria in response to increased stretch (volume overload). Its primary goal is to promote **natriuresis** (sodium excretion) and **diuresis** (water excretion) to reduce blood volume. **Why the Collecting Tubule is correct:** The principal site of action for ANP is the **Inner Medullary Collecting Duct (IMCD)**. ANP binds to its receptors (NPR-A), which increases intracellular **cGMP**. This second messenger inhibits the apical sodium channels (**ENaC**) and the basolateral Na+/K+ ATPase. By blocking sodium reabsorption in this final segment of the nephron, ANP ensures that sodium and water are excreted in the urine. **Why the other options are incorrect:** * **Proximal Tubule:** While some studies suggest minor inhibitory effects on Na-H exchange, the PT is not the primary physiological target for ANP’s natriuretic effect. * **Distal Tubule:** This segment is primarily regulated by Aldosterone and Thiazide-sensitive transporters; it is not the major site for ANP action. * **Henle Loop:** The Thick Ascending Limb is primarily involved in the countercurrent multiplier system and is regulated by different mechanisms (e.g., NKCC2), not ANP. **High-Yield Clinical Pearls for NEET-PG:** 1. **Second Messenger:** ANP and NO (Nitric Oxide) both act via **cGMP**. 2. **Afferent/Efferent Effect:** In the glomerulus, ANP **dilates the afferent arteriole** and **constricts the efferent arteriole**, increasing the GFR (Glomerular Filtration Rate). 3. **Antagonist Role:** ANP acts as a physiological antagonist to the **Renin-Angiotensin-Aldosterone System (RAAS)**. 4. **BNP:** Brain Natriuretic Peptide (secreted by ventricles) acts similarly and is a key clinical marker for diagnosing Heart Failure.

Q: The glomerular filtration rate of the human kidney may be determined by measuring the plasma clearance of which substance?

Inulin. ### Explanation **Why Inulin is the Correct Answer:** To measure the **Glomerular Filtration Rate (GFR)**, a substance must be **freely filtered** at the glomerulus and must be **neither reabsorbed nor secreted** by the renal tubules. **Inulin**, a fructose polymer, perfectly meets these criteria. Since every molecule of inulin filtered into the Bowman’s capsule ends up in the urine, its clearance rate ($C = UV/P$) is exactly equal to the GFR. It is considered the **gold standard** for measuring GFR. **Analysis of Incorrect Options:** * **PAH (Para-aminohippuric acid):** PAH is both filtered and extensively secreted by the tubules. Because it is almost completely cleared from the blood in a single pass through the kidney, its clearance is used to measure **Effective Renal Plasma Flow (ERPF)**, not GFR. * **Urea:** While urea is freely filtered, it undergoes significant **passive reabsorption** (about 50%) in the tubules. Therefore, urea clearance underestimates the true GFR. * **Glucose:** In a healthy individual, glucose is freely filtered but **completely reabsorbed** in the proximal convoluted tubule. Its clearance is normally **zero**, making it useless for measuring GFR. **High-Yield Clinical Pearls for NEET-PG:** * **Creatinine Clearance:** In clinical practice, endogenous creatinine is used to estimate GFR. However, it **slightly overestimates GFR** because a small amount of creatinine is secreted by the tubules. * **Filtration Fraction (FF):** Calculated as $GFR / RPF$. Normal FF is approximately **20%**. * **Criteria for GFR Marker:** Must be non-toxic, not metabolized by the kidney, and not protein-bound.

Question 1

GFR decreases with the following:

Accepted Answer

Hypotension

Answer

Hypoproteinemia

Answer

Hypertension

Answer

All of the above

Question 2

Which of the following substances is primarily filtered by the glomerulus with minimal secretion or reabsorption in the renal tubules?

Accepted Answer

Inulin

Answer

Sodium

Answer

Glucose

Answer

Amino acids

Question 3

Normal filtration fraction is around:

Accepted Answer

0.2

Answer

0.05

Answer

0.1

Answer

None of the above

Question 4

Free water clearance by the kidney is increased by which of the following?

Accepted Answer

Diabetes insipidus

Answer

Renal failure

Answer

Diuretic therapy

Answer

Diabetes mellitus

Question 5

Which of the following is not a site of renal tubular secretion?

Accepted Answer

Collecting tubule

Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Loop of Henle

Question 6

Tamm-Horsfall protein is secreted by epithelial cells of which part of the nephron?

Accepted Answer

Loop of Henle

Answer

Proximal convoluted tubule

Answer

Distal convoluted tubule

Answer

Glomerulus

Question 7

Which of the following substances is reabsorbed along with sodium in the early portion of the proximal tubule?

Accepted Answer

All of the above

Answer

Glucose

Answer

Amino acids

Answer

Bicarbonate

Question 8

Constriction of efferent arteriole produces which of the following effects?

Accepted Answer

All of the above

Answer

Biphasic response on GFR

Answer

Increased peritubular oncotic pressure

Answer

Increased peritubular absorption

Question 9

ANP acts at which part of the nephron?

Accepted Answer

Collecting tubule

Answer

Proximal tubule

Answer

Distal tubule

Answer

Henle loop

Question 10

The glomerular filtration rate of the human kidney may be determined by measuring the plasma clearance of which substance?

Accepted Answer

Inulin

Answer

PAH

Answer

Urea

Answer

Glucose

Renal Physiology — MCQs

Renal Physiology — MCQs

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