Renal Physiology Practice Questions

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.

Q: Which substance(s) is/are freely filtered across glomerular capillaries?

Glucose. **Explanation:** The glomerular filtration barrier acts as a selective sieve based on two primary factors: **molecular size** and **electrical charge**. For a substance to be "freely filtered," its concentration in the Bowman’s space must be equal to its concentration in the plasma (Filtration Ratio = 1.0). **1. Why Glucose is Correct:** Glucose is a small, uncharged molecule with a molecular weight of approximately 180 Da. The glomerular basement membrane allows the passage of all neutral substances with a diameter of less than 4 nm (or <20,000 Da). Since glucose is well below this threshold, it is filtered completely and freely. **2. Analysis of Incorrect Options:** * **Albumin:** Although its molecular radius is small enough to potentially pass, albumin is negatively charged. The glomerular capillary wall (specifically the glycocalyx and podocytes) is lined with **heparan sulfate**, which creates a negative charge barrier that repels albumin. * **Thyroxin:** While thyroxin itself is small, over 99% of it circulates **bound to plasma proteins** (like TBG and albumin). Since large proteins cannot cross the barrier, the bound hormone remains in the blood. * **Creatinine:** *Note:* In many standard physiological contexts, creatinine is also considered freely filtered. However, in the context of this specific question format, glucose is the classic textbook example of a substance that is 100% filtered and then entirely reabsorbed in the proximal tubule. **High-Yield Clinical Pearls for NEET-PG:** * **Minimal Change Disease:** The loss of the negative charge (anionic barrier) leads to massive albuminuria, even though the structural "pores" remain intact. * **Filtration Fraction:** Normally ~20% of renal plasma flow. * **Myoglobin vs. Hemoglobin:** Myoglobin is small and freely filtered (causing renal failure in rhabdomyolysis), whereas Hemoglobin is larger and only filtered if the haptoglobin-binding capacity is exceeded.

Question 1

Which substance undergoes both filtration and reabsorption in the nephron?

Accepted Answer

Glucose

Answer

Insulin

Answer

Creatinine

Answer

Para-aminohippuric acid

Question 2

All of the following cause renal vasoconstriction except?

Accepted Answer

Carbon monoxide

Answer

Angiotensin II

Answer

Norepinephrine

Answer

Endothelin

Question 3

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 4

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 5

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)

Question 6

Glomerular feedback for decreased GFR is mediated by which mechanism?

Accepted Answer

Increase in renin release from the juxtaglomerular complex

Answer

Increased vasopressin activity

Answer

Increased PCT permeability

Answer

Increase in renal sympathetic activity

Question 7

In the micturition reflex, what is the first change to occur?

Accepted Answer

Relaxation of perineal muscles

Answer

Trigone relaxation

Answer

Detrusor contraction

Answer

Decreased urethral pressure

Question 8

Which of the following is defective in renal glucosuria?

Accepted Answer

SGLT-2

Answer

GLUT-1

Answer

GLUT-2

Answer

SGLT-1

Question 9

What is the effect of acute metabolic acidosis on urinary potassium excretion?

Accepted Answer

Decreases urinary potassium excretion

Answer

Has a biphasic effect on potassium excretion

Answer

Does not affect potassium excretion

Answer

Increases urinary potassium excretion

Question 10

Which substance(s) is/are freely filtered across glomerular capillaries?

Accepted Answer

Glucose

Answer

Albumin

Answer

Thyroxin

Answer

Creatinine

Renal Physiology — MCQs

Renal Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?