Sodium and Water Balance — MCQs

10 questions

A patient with SIADH would likely exhibit which electrolyte disturbance?

Vasopressin acts through which aquaporin channels in the collecting duct?

30-year-old male, weighing 70 kg , presents with a serum sodium level of $120 \mathrm{mEq} / \mathrm{L}$. Calculate the total sodium deficit.

Which of the following is NOT a feature of hypernatremia?

In a patient with severe dehydration, which of the following compensatory mechanisms work together to restore blood volume and maintain hemodynamic stability?

A breast fed child presents with hypernatremia (Serum sodium > 170m Eq/L). His urine sodium is 70 mEq/L. Which of the following is the most likely cause –

In a comatose patient with a blood glucose level of 750 mg/dL, which test is most important to perform in addition to serum potassium?

Reduced osmolarity ORS does not contain which of the following ion?

NaCl symporter is present in which part of the nephron?

Q10

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

Sodium and Water Balance Indian Medical PG Practice Questions and MCQs

Question 1: A patient with SIADH would likely exhibit which electrolyte disturbance?

A. Hyperkalemia
B. Hypokalemia
C. Hypernatremia
D. Hyponatremia (Correct Answer)

Explanation: **Hyponatremia** - **SIADH (Syndrome of Inappropriate Antidiuretic Hormone)** causes excessive secretion of ADH, leading to increased free water reabsorption and **dilutional hyponatremia** [1]. - The increased water retention dilutes the body's sodium concentration, resulting in a low serum sodium level [2]. *Hyperkalemia* - **Hyperkalemia** is an elevated potassium level and is not directly caused by SIADH. - While some conditions that cause SIADH might also affect potassium, it is not a direct consequence of ADH excess. *Hypokalemia* - **Hypokalemia**, or low potassium, is typically associated with conditions like diuretic use, vomiting, or diarrhea [2]. - SIADH primarily affects water balance and sodium concentration, not directly potassium levels. *Hypernatremia* - **Hypernatremia** is a high sodium level, which is the opposite of what occurs in SIADH [3]. - It results from conditions causing free water loss or insufficient water intake, not from excess ADH.

Question 2: Vasopressin acts through which aquaporin channels in the collecting duct?

A. Aquaporin 1
B. Aquaporin 2 (Correct Answer)
C. Aquaporin 4
D. Aquaporin 3

Explanation: ***Aquaporin 2*** - Vasopressin (ADH) stimulates the insertion of **Aquaporin 2 (AQP2)** channels into the apical membrane of collecting duct cells, increasing water reabsorption. - This process is crucial for the kidney's ability to concentrate urine and maintain **water balance**. *Aquaporin 1* - **Aquaporin 1 (AQP1)** is predominantly found in the proximal tubules and descending limb of the loop of Henle, where **constitutive water reabsorption** occurs, independent of vasopressin. - It plays a role in bulk water reabsorption rather than regulated fine-tuning. *Aquaporin 3* - **Aquaporin 3 (AQP3)** is located on the **basolateral membrane** of collecting duct cells, facilitating the exit of water from the cell into the interstitial fluid. - While essential for water movement, its insertion into the membrane is **not directly regulated by vasopressin** in the same way as AQP2. *Aquaporin 4* - **Aquaporin 4 (AQP4)** is also found on the **basolateral membrane** of collecting duct cells and in other tissues like the brain. - Similar to AQP3, it allows water to leave the cell but is not the primary target for vasopressin-mediated regulation of water permeability.

Question 3: 30-year-old male, weighing 70 kg , presents with a serum sodium level of $120 \mathrm{mEq} / \mathrm{L}$. Calculate the total sodium deficit.

A. 630 mEq
B. 280 mEq
C. 420 mEq
D. 840 mEq (Correct Answer)
E. 1260 mEq

Explanation: ***840 mEq*** - The formula for calculating **total sodium deficit** is: **(Desired Na - Actual Na) × Total Body Water (TBW)**. - In a male, TBW is approximately **60% of body weight**. For a 70 kg male, **TBW = 0.6 × 70 kg = 42 L**. - With a desired sodium of **140 mEq/L** (normal) and actual sodium of **120 mEq/L**, the total deficit is: - **(140 - 120) × 42 = 20 × 42 = 840 mEq** - This represents the **complete calculated sodium deficit** needed to restore serum sodium to normal levels. - **Note:** In clinical practice, this entire deficit is NOT replaced rapidly. Typically, only **6-12 mEq/L increase per 24 hours** is recommended to prevent **osmotic demyelination syndrome**, but the question asks for the total calculated deficit. *630 mEq* - This value represents a **partial correction target**, corresponding to raising serum sodium to approximately **135 mEq/L** instead of 140 mEq/L: (135 - 120) × 42 = 630 mEq. - Alternatively, it equals about **75% of the total deficit** (840 × 0.75 = 630). - While this may reflect a practical clinical target, it does not answer the question which asks for the **total deficit**. *420 mEq* - This corresponds to raising serum sodium by **10 mEq/L** (10 × 42 = 420 mEq). - This represents the **maximum recommended increase in the first 24 hours** to prevent complications. - It is a safe initial correction amount but not the total calculated deficit. *280 mEq* - This represents an even smaller increment, roughly equivalent to raising serum sodium by **6-7 mEq/L**. - This would be an **ultra-conservative initial correction** for chronic hyponatremia. - It significantly underestimates the total sodium deficit. *1260 mEq* - This is an **overestimation** that might result from incorrectly using 100% body weight as TBW instead of 60%: (140 - 120) × 70 = 1400 mEq (close to this range). - Or from miscalculation using wrong formula components. - This exceeds the actual total sodium deficit.

Question 4: Which of the following is NOT a feature of hypernatremia?

A. Altered mental status
B. Muscle weakness due to hypokalemia (Correct Answer)
C. Elevated intracranial tension
D. Convulsions

Explanation: Muscle weakness due to hypokalemia - **Hypernatremia** does not directly cause **hypokalemia**; these are distinct electrolyte imbalances. Muscle weakness in hypernatremia is more likely due to direct effects on cell excitability rather than low potassium. - While muscle weakness can occur in severe hypernatremia, it is not typically attributed to **hypokalemia**, making this statement inaccurate as a direct feature of hypernatremia. *Convulsions* - Severe **hypernatremia** leads to cellular dehydration, especially in brain cells, which can cause significant neurological symptoms including **convulsions**. - The rapid fluid shifts across the blood-brain barrier due to high extracellular osmolality can disrupt neuronal function, leading to seizures. *Elevated intracranial tension* - **Hypernatremia** causes water to shift out of brain cells into the hyperosmolar extracellular fluid, leading to **brain shrinkage** rather than elevated intracranial tension [1]. - This cellular dehydration reduces brain volume, which would typically lower, not elevate, intracranial tension [1]. *Altered mental status* - **Hypernatremia** causes significant neurological dysfunction due to cellular dehydration in the brain. - This can manifest as various degrees of **altered mental status**, from lethargy and confusion to coma.

Question 5: In a patient with severe dehydration, which of the following compensatory mechanisms work together to restore blood volume and maintain hemodynamic stability?

A. Sympathetic activation
B. ADH release
C. Increased renin secretion
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - In cases of severe dehydration, a coordinated response involving multiple compensatory mechanisms is crucial for restoring **blood volume** and maintaining **hemodynamic stability**. - No single mechanism is sufficient; their combined effects lead to **vasoconstriction**, **fluid retention**, and **increased cardiac output**. *Sympathetic activation* - Leads to **vasoconstriction** of peripheral vessels, increasing **vascular resistance** and shunting blood to vital organs. - Also increases **heart rate** and **contractility**, temporarily sustaining blood pressure and perfusion. *ADH release* - **Antidiuretic hormone (ADH)** increases water reabsorption in the **renal collecting ducts**, reducing urine output and conserving body fluid. - This helps to directly increase **circulating blood volume** by preventing further fluid loss. *Increased renin secretion* - **Renin** initiates the **renin-angiotensin-aldosterone system (RAAS)**, leading to the production of **angiotensin II** and **aldosterone**. - **Angiotensin II** is a potent vasoconstrictor, while **aldosterone** promotes sodium and water reabsorption in the kidneys, both contributing to volume restoration.

Question 6: A breast fed child presents with hypernatremia (Serum sodium > 170m Eq/L). His urine sodium is 70 mEq/L. Which of the following is the most likely cause –

A. Acute tubular necrosis
B. Severe dehydration
C. Excessive intake of sodium (Correct Answer)
D. Diabetes insipidus

Explanation: ***Excessive intake of sodium*** - A critically elevated **serum sodium (>170 mEq/L)** coupled with a high **urine sodium (70 mEq/L)** in a breastfed infant indicates that the kidneys are actively trying to excrete excess sodium. This pattern is consistent with an exogenous sodium overload. - This scenario suggests the ingestion of a **hypertonic solution** or food, likely by mistake, leading to significant sodium toxicity requiring rapid renal excretion. *Acute tubular necrosis* - In ATN, there's impaired renal concentration and reabsorption, but acute kidney injury often leads to **normonatremia or hyponatremia**, not severe hypernatremia. - While urine sodium can be high in ATN due to tubular damage, the primary cause of such extreme hypernatremia would typically be external sodium load. *Severe dehydration* - Severe dehydration usually causes **pre-renal acute kidney injury**, characterized by **high serum sodium** due to water loss, but the kidneys would **conserve sodium**, resulting in a very **low urine sodium** (<20 mEq/L). - The high urine sodium of 70 mEq/L in this case **argues against dehydration** as the primary cause of hypernatremia. *Diabetes insipidus* - Diabetes insipidus (DI) causes **hypernatremia due to free water loss** from the kidneys, resulting in a **dilute urine** with a **low urine osmolality** and typically **low urine sodium**. - The elevated urine sodium of 70 mEq/L is inconsistent with the renal handling of sodium seen in diabetes insipidus.

Question 7: In a comatose patient with a blood glucose level of 750 mg/dL, which test is most important to perform in addition to serum potassium?

A. Serum creatinine
B. Serum sodium
C. Serum ketones
D. Arterial blood gases (Correct Answer)

Explanation: ***Arterial blood gases*** - In a comatose patient with severe hyperglycemia (750 mg/dL), **arterial blood gases (ABGs)** are crucial to assess for **acidosis**, which could indicate **diabetic ketoacidosis (DKA)** or **hyperosmolar hyperglycemic state (HHS)** with lactic acidosis [1], [4]. - The **pH**, **bicarbonate (HCO3-)**, and **pCO2** levels from ABGs help determine the severity and type of metabolic derangement, guiding immediate treatment, especially for potential **cerebral edema** [3], [4]. *Serum creatinine* - While important for assessing **kidney function** in hyperosmolar states, it does not directly evaluate the immediate acid-base status that is critical for neurologic function in a comatose patient. - Renal insufficiency can exacerbate electrolyte imbalances and fluid overload but is secondary to the immediate need for acid-base assessment. *Serum sodium* - **Serum sodium** is important for calculating **effective serum osmolality**, which is elevated in both DKA and HHS, contributing to mental status changes [2]. - However, while important, it does not provide information about the **acid-base balance**, which is a more critical determinant of immediate neurologic stability and treatment in deep coma. *Serum ketones* - **Serum ketones** are essential for distinguishing between **DKA** (high ketones) and **HHS** (low or absent ketones) [4]. - While vital for diagnosis, ketones alone do not give the full picture of **acid-base status** (pH, bicarbonate) which is directly assessed by ABGs and more immediately actionable in managing a severely ill, comatose patient [1].

Question 8: Reduced osmolarity ORS does not contain which of the following ion?

A. Lactate ion (Correct Answer)
B. Potassium ion
C. Citrate ion
D. Sodium ion

Explanation: ***Lactate ion*** - **Reduced osmolarity ORS** (WHO formulation) does **NOT contain lactate**. - The current WHO-ORS uses **trisodium citrate** as the base provider to correct acidosis. - Some older commercial ORS formulations used lactate, but it has been replaced by citrate in standard formulations due to **better stability, palatability, and effectiveness**. *Potassium ion* - **Potassium ions** (20 mmol/L) are an essential component of reduced osmolarity ORS. - They are crucial for replacing electrolyte losses during diarrhea and maintaining **intracellular fluid balance**. *Citrate ion* - **Citrate** (10 mmol/L) is a key component of reduced osmolarity ORS as the base provider. - It helps correct **metabolic acidosis** associated with diarrhea and improves shelf life and taste. *Sodium ion* - **Sodium ions** (75 mmol/L) are vital in reduced osmolarity ORS. - They facilitate the **sodium-glucose co-transport mechanism** across the intestinal wall, promoting optimal water absorption and rehydration.

Question 9: NaCl symporter is present in which part of the nephron?

A. PCT
B. DCT (Correct Answer)
C. Loop of Henle
D. Collecting duct

Explanation: ***DCT*** - The **NaCl symporter** (also known as the **Na-Cl co-transporter** or NCC) is located in the **luminal membrane** of cells in the **distal convoluted tubule (DCT)**. - This transporter is responsible for reabsorbing approximately 5-10% of filtered sodium and chloride, and it is the target of **thiazide diuretics**. *PCT* - The **proximal convoluted tubule (PCT)** is responsible for bulk reabsorption of Na+ through various mechanisms, including Na+/H+ exchangers and Na+-glucose co-transporters, but not the specific NaCl symporter found in the DCT. - While significant NaCl reabsorption occurs here, it is primarily driven by different transport proteins. *Loop of Henle* - The **thick ascending limb of the loop of Henle** uses the **Na-K-2Cl co-transporter (NKCC2)** for Na+ reabsorption, which is distinct from the NaCl symporter. - This segment is the target for **loop diuretics**. *Collecting duct* - The **collecting duct** reabsorbs Na+ primarily through the **epithelial sodium channel (ENaC)**, which is regulated by aldosterone. - While reabsorption of sodium occurs, the specific NaCl symporter is not present in this segment.

Question 10: 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.

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free