Sodium and Water Balance — MCQs

10 questions

Which of the following is most important in sodium and water retention ?

A healthy 22-year-old female medical student with normal kidneys decreases her sodium intake by 50% for a period of 2 months. Which of the following parameters is expected to increase in response to the reduction in sodium intake?

All of the following are true about the action of ADH, except:

In a patient who has diarrhoea and vomiting with inadequate water intake is suffering from –

In which of the following conditions is blood osmolality increased?

Interstitial fluid volume can be determined by:

The normal range of serum osmolality (in mosm/L) is:

The body fluid compartments of a patient were measured, showing the following ion concentrations: - Sodium (Na): $10 \mathrm{mEq} / \mathrm{L}$ - Potassium (K): $140 \mathrm{mEq} / \mathrm{L}$ - Chloride (Cl): $15 \mathrm{mEq} / \mathrm{L}$ Based on these values, which fluid compartment is being described?

Interstitial fluid volume can be measured by:

Q10

Normal anion gap is___ mmol/L?

Sodium and Water Balance Indian Medical PG Practice Questions and MCQs

Question 1: Which of the following is most important in sodium and water retention ?

A. Renin angiotensin system (Correct Answer)
B. ANP
C. BNP
D. Vasopressin

Explanation: ***Renin angiotensin system*** - The **renin-angiotensin-aldosterone system (RAAS)** is the most important mechanism for **both sodium AND water retention**, which is what the question specifically asks about. - **Aldosterone** directly promotes **sodium reabsorption** in the principal cells of the collecting duct by increasing apical ENaC channels and basolateral Na-K-ATPase pumps. - **Angiotensin II** stimulates sodium reabsorption in the proximal tubule and also stimulates ADH release, contributing to water retention. - When sodium is retained, **water follows passively** due to the osmotic gradient, resulting in effective volume expansion. - RAAS is the primary system activated in states of volume depletion and is most important for combined sodium and water retention. *Vasopressin* - **Vasopressin (ADH)** primarily controls **water retention only** by increasing aquaporin-2 channels in the collecting duct. - While crucial for water balance, it has minimal direct effect on sodium reabsorption. - It causes retention of **free water**, which can actually dilute plasma sodium concentration. - ADH is the answer if the question asked about water retention alone, but not for combined sodium and water retention. *ANP* - **Atrial natriuretic peptide (ANP)** promotes **sodium and water excretion** (natriuresis and diuresis). - Released in response to atrial stretch from volume expansion. - Acts to *oppose* retention mechanisms, making it incorrect for this question. *BNP* - **Brain natriuretic peptide (BNP)** similarly promotes **natriuresis and diuresis**. - Released from ventricular myocytes in response to volume overload. - Like ANP, it acts to *excrete* sodium and water, not retain them.

Question 2: A healthy 22-year-old female medical student with normal kidneys decreases her sodium intake by 50% for a period of 2 months. Which of the following parameters is expected to increase in response to the reduction in sodium intake?

A. Atrial natriuretic peptide release
B. Extracellular fluid volume
C. Arterial pressure
D. Renin release (Correct Answer)

Explanation: ***Renin release*** - A reduction in **sodium intake** leads to decreased extracellular fluid volume and **reduced renal perfusion pressure**, which stimulates **renin release** from the juxtaglomerular cells. - Renin initiates the **renin-angiotensin-aldosterone system (RAAS)**, leading to **angiotensin II** formation and increased **aldosterone** secretion, aimed at sodium and water retention. *Atrial natriuretic peptide release* - **Atrial natriuretic peptide (ANP)** release is stimulated by **atrial stretch** due to increased blood volume and pressure, which would decrease with reduced sodium intake. - Therefore, ANP release would likely **decrease** or remain unchanged, not increase, in response to chronic sodium restriction. *Extracellular fluid volume* - A decrease in sodium intake directly leads to a **reduction in total body sodium**, which is the primary determinant of **extracellular fluid volume**. - The body attempts to maintain fluid balance, but chronic sodium restriction will ultimately lead to a **decrease** in extracellular fluid volume as the kidneys excrete less water to match the lower sodium intake. *Arterial pressure* - Reduced sodium intake typically leads to a **decrease in extracellular fluid volume** and **cardiac output**, which in turn causes a **reduction in arterial blood pressure**. - The activation of the RAAS aims to mitigate this drop but usually does not fully compensate to increase pressure above baseline in this scenario.

Question 3: All of the following are true about the action of ADH, except:

A. Acts on collecting ducts and increases water permeability
B. Secreted by neurosecretion from posterior pituitary
C. Post-operative increase in secretion
D. Increased secretion when plasma osmolality is low (Correct Answer)

Explanation: ***Increased secretion when plasma osmolality is low*** - **Antidiuretic hormone (ADH)** secretion is *inhibited* when plasma osmolality is low. - ADH is secreted to conserve water and *increase* plasma osmolality when it is too high, or plasma volume is too low. - Normal osmolality range is 280-290 mOsm/kg; ADH secretion increases above this threshold. *Acts on collecting ducts and increases water permeability* - This statement is true; ADH binds to **V2 receptors** on the principal cells of the collecting ducts. - This binding leads to the insertion of **aquaporin-2 channels** into the apical membrane, increasing water reabsorption. *Secreted by neurosecretion from posterior pituitary* - This statement is true; ADH is synthesized in the **hypothalamus** (supraoptic and paraventricular nuclei) and transported down nerve axons. - It is then stored in and released from the **posterior pituitary gland**, a process known as neurosecretion. *Post-operative increase in secretion* - This statement is true; surgical stress, pain, and common postoperative medications (e.g., narcotics) can stimulate ADH release. - This can lead to **hyponatremia** and fluid retention in the postoperative period due to excessive free water reabsorption.

Question 4: In a patient who has diarrhoea and vomiting with inadequate water intake is suffering from –

A. Extracellular dehydration with hypernatremia
B. Intracellular dehydration with hyponatremia
C. Intracellular dehydration with hypernatremia
D. Extracellular dehydration with hyponatremia (Correct Answer)

Explanation: ***Extracellular dehydration with hyponatremia*** - Diarrhea and vomiting primarily lead to a loss of **isotonic fluid** from the **extracellular space**, resulting in **extracellular dehydration** (hypovolemia) [1]. - While both water and sodium are lost, the replacement with inadequate water intake or plain water can dilute the remaining extracellular fluid, leading to relatively more water than sodium, thus causing **hyponatremia** [1]. *Extracellular dehydration with hypernatremia* - Although there is **extracellular dehydration** due to fluid loss, **hypernatremia** would occur if water loss significantly exceeded sodium loss, or if the water deficit was not adequately replaced, leading to a concentration of existing sodium. - In this scenario, the combination of fluid loss and insufficient water intake is more likely to cause a relative decrease in sodium concentration or an isotonic loss leading to subsequent hyponatremia [1]. *Intracellular dehydration with hyponatremia* - **Intracellular dehydration** occurs primarily due to a shift of water out of cells into the extracellular space, often as a result of **hypernatremia** in the extracellular fluid. - **Hyponatremia** would typically cause water to shift *into* cells, leading to cellular swelling, not intracellular dehydration [1]. *Intracellular dehydration with hypernatremia* - **Intracellular dehydration** is indeed often associated with **hypernatremia** in the extracellular fluid, as the increased extracellular osmolarity pulls water out of the cells [1]. - However, the primary effect of diarrhea and vomiting is directly on the **extracellular fluid volume**, and the subsequent alterations in sodium concentration are more complex than simple intracellular dehydration with hypernatremia.

Question 5: In which of the following conditions is blood osmolality increased?

A. SIADH
B. Psychogenic polydipsia
C. Diarrhea (Correct Answer)
D. Cerebral toxoplasmosis

Explanation: ***Diarrhea*** - Diarrhea leads to a significant loss of **water and electrolytes** from the body, primarily from the extracellular fluid compartment. - This imbalance causes **hemoconcentration** and an increase in the concentration of solutes in the blood, thereby raising blood osmolality. *SIADH* - **Syndrome of Inappropriate Antidiuretic Hormone (SIADH)** is characterized by excessive secretion of ADH, leading to **dilutional hyponatremia**. - The excess water retention dilutes the blood, resulting in **decreased serum osmolality**. *Psychogenic polydipsia* - This condition involves excessive water intake due to psychological factors, which causes **dilution of body fluids**. - The increased water volume without a proportional increase in solutes leads to **decreased plasma osmolality**. *Cerebral toxoplasmosis* - **Cerebral toxoplasmosis** is an opportunistic infection of the brain, typically seen in immunocompromised individuals. - It primarily causes neurological symptoms and **does not directly impact blood osmolality** unless complicated by other factors like dehydration or SIADH (which is not a primary effect).

Question 6: Interstitial fluid volume can be determined by:

A. Radioactive iodine and radiolabelled water
B. Radioactive sodium and radioactive water
C. Radioactive sodium and radioactive labelled albumin (Correct Answer)
D. Radioactive water and radiolabelled albumin

Explanation: ***Radioactive sodium and radioactive labelled albumin*** - **Interstitial fluid volume** (ISF) is the difference between **extracellular fluid** (ECF) and **plasma volume**. - **Radioactive sodium** can be used to estimate ECF, and **radioactive labelled albumin** can be used to estimate plasma volume. *Radioactive iodine and radiolabelled water* - **Radioactive iodine** (often as iodide) is used for **extracellular fluid** (ECF) measurement, not directly for ISF alone. - **Radiolabelled water** (e.g., tritiated water) is used to measure **total body water** (TBW), which includes intracellular and extracellular components. *Radioactive sodium and radioactive water* - **Radioactive sodium** is used to measure **extracellular fluid** (ECF) due to its limited entry into cells. - **Radioactive water** (e.g., tritiated water) measures **total body water** (TBW), not specifically interstitial fluid. *Radioactive water and radiolabelled albumin* - **Radioactive water** measures **total body water** (TBW), which encompasses all fluid compartments. - **Radiolabelled albumin** measures **plasma volume** because albumin remains within the vascular space.

Question 7: The normal range of serum osmolality (in mosm/L) is:

A. 200 to 250
B. 280 to 295 (Correct Answer)
C. 300 to 320
D. 350 to 375

Explanation: ***280 to 295*** - This range represents the **physiological concentration** of solutes in the blood, primarily determined by sodium, glucose, and urea. - Maintaining osmolality within this **narrow range** is crucial for proper cellular function and fluid balance. *200 to 250* - A serum osmolality in this range would indicate **hypotonicity**, leading to water movement into cells and potential **cellular swelling**. - Values this low are typically seen in conditions like **severe hyponatremia** or excessive water intake. *300 to 320* - While slightly above the normal range, this might be seen in cases of **mild dehydration** or conditions like uncontrolled diabetes where blood glucose is elevated. - Sustained levels in this range indicate increased solute concentration, which can lead to **cellular dehydration**. *350 to 375* - This range represents significantly elevated serum osmolality, indicating **hypertonic states** such as severe dehydration, **hyperglycemic hyperosmolar state (HHS)**, or severe hypernatremia. - Such high osmolality would result in substantial **cellular shrinkage** and can be life-threatening.

Question 8: The body fluid compartments of a patient were measured, showing the following ion concentrations: - Sodium (Na): $10 \mathrm{mEq} / \mathrm{L}$ - Potassium (K): $140 \mathrm{mEq} / \mathrm{L}$ - Chloride (Cl): $15 \mathrm{mEq} / \mathrm{L}$ Based on these values, which fluid compartment is being described?

A. Plasma
B. ICF (Correct Answer)
C. Interstitial fluid
D. ECF

Explanation: ***ICF*** - The measured ion concentrations, especially **high potassium (140 mEq/L)** and **low sodium (10 mEq/L)**, are characteristic of the **intracellular fluid (ICF)**, where potassium is the primary cation and sodium is kept low by the Na+/K+-ATPase pump. - **Chloride levels (15 mEq/L)** are also significantly lower in the ICF compared to extracellular fluids. *Plasma* - Plasma typically has **high sodium (around 140 mEq/L)** and **low potassium (around 4 mEq/L)**, which contradicts the given measurements. - Chloride levels in plasma are usually much higher, around **100-105 mEq/L**. *Interstitial fluid* - Interstitial fluid has an electrolyte composition very similar to plasma, with **high sodium** and **low potassium**, differing mainly in protein content. - This composition is not consistent with the given measurements. *ECF* - The ECF (extracellular fluid), which includes both plasma and interstitial fluid, is characterized by **high sodium** and **low potassium**. - The given ion concentrations, particularly the very **high potassium** and **low sodium**, are directly opposite to the typical ECF profile.

Question 9: Interstitial fluid volume can be measured by:

A. Tritium oxide - Sodium thiosulfate
B. Inulin - Serum albumin labelled with radioactive Iodine (Correct Answer)
C. Inulin - Radioactive sodium
D. Aminopyrine - Sucrose

Explanation: ***Inulin - Serum albumin labelled with radioactive Iodine*** - The **interstitial fluid volume** is calculated by subtracting the plasma volume from the extracellular fluid volume. - **Inulin** is used to measure **extracellular fluid volume** because it freely distributes throughout the extracellular space but does not enter cells. - **Serum albumin labeled with radioactive iodine** measures **plasma volume** as it stays primarily within the bloodstream due to its large size. *Tritium oxide - Sodium thiosulfate* - **Tritium oxide** (or D2O) is used to measure **total body water (TBW)**, as it distributes throughout all fluid compartments. - **Sodium thiosulfate** is used to measure **extracellular fluid volume**, similar to inulin. *Inulin - Radioactive sodium* - While **inulin** measures **extracellular fluid volume**, **radioactive sodium** (typically 24Na) also measures extracellular fluid volume but can slightly overestimate it due to slow intracellular penetration. - This combination doesn't directly provide a method for exclusively calculating interstitial fluid by subtraction from plasma volume. *Aminopyrine - Sucrose* - **Aminopyrine** is primarily used to measure the **volume of distribution of specific drugs** or gastric acid secretion, not fluid compartments. - **Sucrose** can be used to measure **extracellular fluid volume** as it does not readily cross cell membranes, similar to inulin, but it's not the primary combination for measuring interstitial fluid from the given options.

Question 10: Normal anion gap is___ mmol/L?

A. 8-16 (Correct Answer)
B. 30-34
C. 20-24
D. 0-4

Explanation: ***8-16*** - The normal range for the **anion gap** is generally considered to be 8-16 mmol/L, reflecting the unmeasured anions in the plasma. - This range can vary slightly between laboratories, but **8-16 mmol/L** is the most commonly accepted range in clinical practice. *30-34* - This range is significantly **higher than normal** and would indicate a **high anion gap metabolic acidosis**, rather than a normal anion gap. - A high anion gap suggests an accumulation of **unmeasured acids** in the body, such as in lactic acidosis or ketoacidosis. *20-24* - This value is also **elevated** compared to the normal range, suggesting a high anion gap. - An anion gap in this range would prompt investigation into causes of **metabolic acidosis** with an increased anion gap. *0-4* - This range is significantly **lower than normal** and could indicate a **low or negative anion gap**, which is a rare finding. - A low anion gap is often associated with conditions like **hypoalbuminemia**, multiple myeloma (due to paraproteins), or severe hypernatremia.

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