Renal Physiology Practice Questions

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.

Q: Ultrafiltrate passes through all filtration barriers except?

Fenestra in podocytes. The glomerular filtration barrier is a highly specialized trilaminar structure that filters blood to form urine. To understand why **Option C** is the correct answer, one must distinguish between the physical structures of the barrier and the terminology used to describe them. ### Explanation of the Correct Answer The glomerular filtration barrier consists of three layers: the fenestrated endothelium, the basement membrane, and the podocyte layer. While the **endothelial cells** have "fenestrae" (pores), **podocytes do not have fenestrae**. Instead, podocytes have long finger-like projections called **pedicels (foot processes)**. The spaces between these interdigitating pedicels are called **filtration slits** (bridged by a slit diaphragm). Therefore, "Fenestra in podocytes" is a non-existent anatomical feature, making it the correct choice for what the ultrafiltrate does *not* pass through. ### Analysis of Incorrect Options * **A. Endothelial fenestra:** These are large pores (70–100 nm) in the glomerular capillary wall that allow all non-cellular components of blood to pass through. * **B. Basement membrane:** This is the middle layer composed of Type IV collagen and heparan sulfate. It acts as a physical and charge-selective barrier (negative charge). * **D. Filtration slit between podocytes:** These are the final gaps (approx. 25–40 nm) through which the filtrate enters Bowman’s space. ### NEET-PG High-Yield Pearls * **Charge Barrier:** The glomerular barrier is negatively charged due to **heparan sulfate** and **sialoproteins** (like podocalyxin). This repels negatively charged proteins like albumin. * **Nephrin:** This is the key protein in the slit diaphragm. Mutations in the NPHS1 gene (encoding nephrin) lead to **Finnish-type congenital nephrotic syndrome**. * **Minimal Change Disease:** Characterized by the **effacement (flattening) of podocyte foot processes**, leading to massive proteinuria.

Q: In the proximal tubule, bicarbonate (HCO3-) reabsorption occurs via which mechanism?

Sodium-hydrogen exchanger. **Explanation:** In the proximal convoluted tubule (PCT), approximately 80–90% of filtered bicarbonate ($HCO_3^-$) is reabsorbed. This process is indirect and relies on the **Sodium-Hydrogen Exchanger (NHE3)** located on the apical membrane. **Mechanism:** 1. The NHE3 transporter pumps $H^+$ ions into the tubular lumen in exchange for $Na^+$ ions. 2. In the lumen, $H^+$ combines with filtered $HCO_3^-$ to form carbonic acid ($H_2CO_3$). 3. **Carbonic Anhydrase (Type IV)** on the brush border dissociates $H_2CO_3$ into $CO_2$ and $H_2O$. 4. $CO_2$ diffuses into the cell, where **Carbonic Anhydrase (Type II)** converts it back into $HCO_3^-$ and $H^+$. 5. The $HCO_3^-$ then exits the basolateral membrane into the blood via the $Na^+$-$HCO_3^-$ symporter (NBCe1). **Analysis of Incorrect Options:** * **Option A (NCC):** Located in the **Distal Convoluted Tubule**. It is the target of Thiazide diuretics. * **Option B (NKCC2):** Located in the **Thick Ascending Limb** of the Loop of Henle. It is the target of Loop diuretics (Furosemide). * **Option D ($Na^+$-$K^+$ Exchanger):** While the $Na^+$-$K^+$ ATPase exists on the basolateral membrane to create the gradient for $Na^+$, it is not the primary mechanism for bicarbonate reabsorption. **High-Yield Clinical Pearls for NEET-PG:** * **Acetazolamide:** Inhibits Carbonic Anhydrase, leading to bicarbonate loss in urine (Proximal Renal Tubular Acidosis/Type 2 RTA). * **Angiotensin II:** Stimulates the NHE3 exchanger, increasing $HCO_3^-$ reabsorption (explaining contraction alkalosis). * **Rate-limiting step:** The secretion of $H^+$ via the NHE3 is the primary driver for $HCO_3^-$ reclamation.

Q: What is the greatest stimulator for ADH secretion?

Hyperosmolarity. **Explanation:** **1. Why Hyperosmolarity is Correct:** Antidiuretic Hormone (ADH), or Vasopressin, is primarily regulated by two mechanisms: osmotic and hemodynamic. **Hyperosmolarity** is the **most sensitive** and potent stimulator. Osmoreceptors in the hypothalamus (OVLT and SFO) can detect changes in plasma osmolarity as small as **1%**. When osmolarity rises above the threshold (approx. 280–285 mOsm/L), ADH is released to promote water reabsorption in the renal collecting ducts via V2 receptors. **2. Why Other Options are Incorrect:** * **Hypovolemia (D) and Hypotension (C):** These are potent stimulators but are **less sensitive** than osmolarity. A significant decrease in blood volume or pressure (usually **>10%**) is required to trigger ADH release via baroreceptors. While hypovolemia can override osmolarity in extreme cases (e.g., severe hemorrhage), hyperosmolarity remains the "greatest" (most precise and frequent) physiological stimulator. * **Hyponatremia (B):** This actually **inhibits** ADH secretion. Low sodium levels (hypo-osmolarity) signal the body to excrete excess water to restore osmotic balance. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sensitivity vs. Potency:** Osmolarity is the most *sensitive* trigger (1% change), while Hypovolemia/Hypotension is a more *potent* trigger during emergencies (can cause ADH levels to rise 10x higher than osmotic triggers). * **V1 vs. V2 Receptors:** V1 receptors cause vasoconstriction (at high concentrations), while V2 receptors (via Aquaporin-2) mediate the antidiuretic effect. * **SIADH:** Characterized by excessive ADH despite hyponatremia and low plasma osmolarity. * **Diabetes Insipidus:** Central (lack of ADH) or Nephrogenic (resistance to ADH) leads to polyuria and hyperosmolarity.

Question 1

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 2

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 3

Which of the following is defective in renal glucosuria?

Accepted Answer

SGLT-2

Answer

GLUT-1

Answer

GLUT-2

Answer

SGLT-1

Question 4

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 5

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

Accepted Answer

Glucose

Answer

Albumin

Answer

Thyroxin

Answer

Creatinine

Question 6

Ultrafiltrate passes through all filtration barriers except?

Accepted Answer

Fenestra in podocytes

Answer

Endothelial fenestra

Answer

Basement membrane

Answer

Filtration slit between podocytes

Question 7

Which statement is false regarding the ascending limb of the loop of Henle?

Accepted Answer

A defect in the Na+K+2Cl- channel leads to Pendred syndrome.

Answer

It is highly permeable to sodium.

Answer

It is permeable to magnesium.

Answer

It is a part of the countercurrent multiplier system.

Question 8

In the proximal tubule, bicarbonate (HCO3-) reabsorption occurs via which mechanism?

Accepted Answer

Sodium-hydrogen exchanger

Answer

Sodium-chloride cotransporter

Answer

Sodium-potassium-2-chloride cotransporter

Answer

Sodium-potassium exchanger

Question 9

What is the greatest stimulator for ADH secretion?

Accepted Answer

Hyperosmolarity

Answer

Hyponatremia

Answer

Hypotension

Answer

Hypovolemia

Question 10

Which characteristic allows a particle to pass easily through the glomerular membrane, assuming it has the same diameter as another particle?

Accepted Answer

Positive charge

Answer

Negative charge

Answer

Neutral charge

Answer

Charge has no relation to glomerular filtration

Renal Physiology — MCQs

Renal Physiology — MCQs

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