Renal Physiology Practice Questions

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).

Q: At what age does glomerular filtration rate typically begin to decline?

30-40 years. **Explanation:** The Glomerular Filtration Rate (GFR) reaches its peak adult levels (approximately 125 mL/min/1.73m²) by the age of 2 years. It remains relatively stable throughout young adulthood. However, starting between the ages of **30 and 40 years**, a progressive physiological decline in GFR begins. **Why 30-40 years is correct:** The decline is primarily due to the natural aging process of the kidneys, characterized by a gradual loss of functioning nephrons (nephrosclerosis), reduction in renal blood flow, and cortical atrophy. On average, the GFR decreases at a rate of approximately **0.8 to 1.0 mL/min/1.73m² per year** after the age of 40. **Analysis of Incorrect Options:** * **A (10-15 years):** At this stage, renal function is at its physiological peak. The kidneys are still growing or maintaining maximum efficiency. * **C & D (45-65 years):** While the decline becomes more clinically evident and measurable in these age groups, the *initiation* of the decline occurs much earlier (in the 4th decade of life). **High-Yield Clinical Pearls for NEET-PG:** * **Formula for Age-Related Decline:** A common rule of thumb is that GFR decreases by ~10 mL/min per decade after age 40. * **Creatinine Paradox:** Despite the decrease in GFR, serum creatinine levels often remain within the "normal" range in the elderly. This is because muscle mass (the source of creatinine) also decreases with age, masking the decline in renal clearance. * **Cockcroft-Gault Equation:** This formula specifically incorporates **age** as a variable in the denominator, reflecting its inverse relationship with GFR. * **Clinical Significance:** Dose adjustment for renally cleared drugs (e.g., Aminoglycosides, Digoxin) is crucial in elderly patients, even if their serum creatinine appears normal.

Q: What is the tonicity of the fluid in the distal convoluted tubule (DCT)?

Hypotonic. **Explanation:** The tonicity of the tubular fluid changes significantly as it passes through different segments of the nephron. The fluid entering the **Distal Convoluted Tubule (DCT)** is characteristically **hypotonic** (approximately 100 mOsm/L) compared to plasma (290 mOsm/L). **Why Option A is Correct:** This occurs because of the preceding action in the **Thick Ascending Limb (TAL)** of the Loop of Henle. The TAL is known as the **"diluting segment"** of the nephron. It actively reabsorbs solutes (via the $Na^+-K^+-2Cl^-$ symporter) but is strictly impermeable to water. As salt leaves the tubule without water following it, the luminal fluid becomes increasingly dilute. By the time it reaches the early DCT, the fluid is significantly more dilute than the surrounding interstitium and plasma. **Why Other Options are Incorrect:** * **Option B (Hypertonic):** Fluid is hypertonic only at the tip of the Loop of Henle (up to 1200 mOsm/L) due to the countercurrent multiplier system. * **Option C (Isotonic):** Fluid is isotonic (300 mOsm/L) in the **Proximal Convoluted Tubule (PCT)**, where water and solutes are reabsorbed in equal proportions (obligatory water reabsorption). **High-Yield Clinical Pearls for NEET-PG:** * **Early DCT:** Continues the dilution process via the **$Na^+-Cl^-$ cotransporter** (the target of **Thiazide diuretics**). It remains impermeable to water. * **Late DCT & Collecting Duct:** Tonicity here is **variable** and depends entirely on **Antidiuretic Hormone (ADH)**. In the presence of ADH, water is reabsorbed, making urine hypertonic; in its absence, urine remains hypotonic. * **Macula Densa:** Located at the junction of the TAL and DCT, it senses $Cl^-$ levels to regulate the Glomerular Filtration Rate (Tubuloglomerular feedback).

Question 1

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 2

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 3

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 4

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 5

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

Question 6

At what age does glomerular filtration rate typically begin to decline?

Accepted Answer

30-40 years

Answer

10-15 years

Answer

45-55 years

Answer

55-65 years

Question 7

In a patient with severe central diabetes insipidus caused by a lack of antidiuretic hormone secretion, which part of the nephron would have the lowest tubular fluid osmolarity?

Accepted Answer

Collecting duct

Answer

Proximal convoluted tubule

Answer

Loop of Henle (descending limb)

Answer

Loop of Henle (ascending limb)

Question 8

What is the tonicity of the fluid in the distal convoluted tubule (DCT)?

Accepted Answer

Hypotonic

Answer

Hypertonic

Answer

Isotonic

Answer

None of the above

Question 9

Angiotensin-II mainly acts by which mechanism on the glomerulus?

Accepted Answer

Efferent arteriole constriction

Answer

Afferent arteriole dilatation

Answer

Efferent arteriole dilatation

Answer

Afferent arteriole constriction

Question 10

Concentrating ability of the kidney is impaired if:

Accepted Answer

Blood flow through the vasa recta is increased substantially

Answer

There is damage to the glomerulus

Answer

There is excess secretion of ADH

Answer

There is vigorous activity of the Na+-K+-pump

Renal Physiology — MCQs

Renal Physiology — MCQs

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