Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 51: What is the osmotic pressure of 1 mol of an ideal solute relative to pure water?

A. 6.5 atm
B. 22.4 atm (Correct Answer)
C. 4 atm
D. 2 atm

Explanation: ### Explanation **1. Why the Correct Answer is Right (The Concept)** The osmotic pressure of a solution is determined by the number of solute particles present, as described by **van't Hoff’s Law**. According to the Ideal Gas Law ($PV = nRT$), at standard temperature and pressure (STP, 0°C or 273 K), one mole of an ideal gas occupies a volume of 22.4 liters at 1 atmosphere of pressure. In the context of solutions, if **1 mole** of an ideal (non-dissociating) solute is dissolved in **1 liter** of water at 0°C, it exerts an osmotic pressure of **22.4 atmospheres (atm)**. This is a fundamental constant in physiology used to calculate the tonicity of body fluids. **2. Why the Incorrect Options are Wrong** * **Option A (6.5 atm):** This value is incorrect. However, it is worth noting that the total osmotic pressure of human plasma is approximately 7.3 atm (5500-5600 mmHg), which is much lower than 22.4 atm because plasma is not a 1-molar solution. * **Options C & D (4 atm and 2 atm):** These are arbitrary values that do not correspond to any standard physical constants related to molarity and osmotic pressure at STP. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Osmolarity vs. Osmolality:** In clinical practice, we use **Osmolality** (mOsm/kg of water) because it is independent of temperature, whereas Osmolarity (mOsm/L) changes with temperature. * **Plasma Osmolality:** Normal range is **280–295 mOsm/kg**. It is primarily determined by Sodium ($Na^+$), Glucose, and BUN. * **Formula:** $Calculated\ Osmolality = 2[Na^+] + \frac{Glucose}{18} + \frac{BUN}{2.8}$. * **Oncotic Pressure:** While the total osmotic pressure of plasma is high (~5500 mmHg), the **Colloid Osmotic Pressure (Oncotic Pressure)** exerted by proteins (mainly albumin) is only about **25–28 mmHg**. This small fraction is crucial for preventing edema.

Question 52: Calculate the plasma osmolality given serum Na+ is 130 mEq/L, glucose is 180 mg/dL, BUN is 56 mg/dL, K+ is 5 mEq/L, and Cl- is 117 mEq/L.

A. 260 mOsm/kg
B. 270 mOsm/kg
C. 280 mOsm/kg
D. 290 mOsm/kg (Correct Answer)

Explanation: **Explanation:** The plasma osmolality is calculated using the standard clinical formula, which accounts for the primary solutes contributing to osmotic pressure in the extracellular fluid: **Formula:** $\text{Plasma Osmolality} = 2 \times [\text{Na}^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}$ **Calculation:** 1. **Sodium component:** $2 \times 130 = 260$ (Sodium is doubled to account for associated anions like $Cl^-$ and $HCO_3^-$). 2. **Glucose component:** $180 / 18 = 10$ 3. **BUN component:** $56 / 2.8 = 20$ 4. **Total:** $260 + 10 + 20 = \mathbf{290\text{ mOsm/kg}}$ **Why other options are incorrect:** * **Option A (260):** Only accounts for Sodium and its anions, ignoring glucose and urea. * **Option B (270):** Likely misses the BUN contribution or uses an incorrect divisor. * **Option C (280):** Incorrectly calculates the contribution of glucose or urea. * **Note on $K^+$ and $Cl^-$:** These are not added separately in the standard formula. $Cl^-$ is accounted for by doubling the $Na^+$, and $K^+$ is an intracellular cation with negligible impact on plasma osmolality calculations. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Range:** 275–295 mOsm/kg. * **Osmolar Gap:** The difference between measured osmolality (via freezing point depression) and calculated osmolality. A gap **>10 mOsm/L** suggests the presence of unmeasured osmotically active substances (e.g., Ethanol, Methanol, Ethylene glycol). * **Effective Osmolality (Tonicity):** Calculated as $2 \times [Na^+] + \text{Glucose}/18$. Urea is excluded because it is an "ineffective osmole" that freely crosses cell membranes and does not cause water shifts.

Question 53: Which of the following statements about sodium is true?

A. Normal serum level is 135-145 mEq/L
B. Daily intake is 150 mmol of NaCl
C. Major portion is extracellular
D. All of the above (Correct Answer)

Explanation: **Explanation:** Sodium ($Na^+$) is the principal cation of the extracellular fluid (ECF) and plays a critical role in maintaining osmotic pressure and fluid balance. 1. **Normal Serum Level (Option A):** The physiological range for serum sodium is **135–145 mEq/L**. Values below this range indicate hyponatremia, while values above indicate hypernatremia. 2. **Daily Intake (Option B):** The average dietary intake of sodium chloride (NaCl) is approximately **100–150 mmol/day** (roughly 5–8 grams of salt). The kidneys maintain homeostasis by excreting an equivalent amount to match this intake. 3. **Distribution (Option C):** Sodium is the **major extracellular cation**. Approximately 90–95% of the body's sodium is located in the ECF, while intracellular concentrations are kept low (around 10–14 mEq/L) by the active **Na⁺-K⁺ ATPase pump**. Since all three statements are physiologically accurate, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Sodium and Osmolality:** Sodium is the primary determinant of plasma osmolality. Estimated Plasma Osmolality = $2 \times [Na^+] + [Glucose]/18 + [BUN]/2.8$. * **Regulation:** Sodium excretion is primarily regulated by **Aldosterone** (increases reabsorption in the distal tubule) and **Atrial Natriuretic Peptide (ANP)** (increases excretion). * **Hyponatremia Correction:** Rapid correction of chronic hyponatremia can lead to **Osmotic Demyelination Syndrome** (Central Pontine Myelinolysis). The safe rate of correction is generally $<8–10$ mEq/L in 24 hours.

Question 54: Which of the following ions is present in the greatest amount to determine the volume of extracellular fluid?

A. Na+ (Correct Answer)
B. K+
C. Cl-
D. Ca2+

Explanation: **Explanation:** The volume of the **Extracellular Fluid (ECF)** is primarily determined by the total amount of osmotically active solutes it contains. **Sodium (Na+)** is the correct answer because it is the most abundant cation in the ECF (normal range: 135–145 mEq/L). According to the principle of osmosis, "water follows salt." Since sodium and its associated anions (primarily chloride and bicarbonate) account for over 90% of ECF osmolarity, the total body sodium content is the main determinant of ECF volume. **Analysis of Incorrect Options:** * **B. K+ (Potassium):** This is the primary **intracellular** cation. While it determines intracellular fluid (ICF) volume, its ECF concentration is very low (3.5–5.0 mEq/L), making its contribution to ECF volume negligible. * **C. Cl- (Chloride):** Although chloride is the most abundant anion in the ECF, its movement is usually passive and secondary to sodium to maintain electroneutrality. Sodium is considered the primary "driver." * **D. Ca2+ (Calcium):** Calcium is present in very small concentrations in the ECF (8.5–10.5 mg/dL) and functions mainly in cell signaling, bone mineralization, and coagulation rather than volume regulation. **High-Yield Clinical Pearls for NEET-PG:** * **Gibbs-Donnan Effect:** Explains why plasma has a slightly higher protein concentration and different electrolyte distribution than interstitial fluid. * **Osmolarity vs. Tonicity:** Sodium is an "effective osmole" because it does not easily cross cell membranes, thus exerting an osmotic pressure that keeps water in the ECF. * **Clinical Correlation:** Disorders of sodium *concentration* (Hyponatremia/Hypernatremia) usually reflect issues with **water balance**, whereas disorders of total sodium *content* reflect issues with **ECF volume** (Edema or Dehydration).

Question 55: Major contribution to plasma osmolality is by which ion?

A. Sodium (Correct Answer)
B. Potassium
C. Glucose
D. Calcium

Explanation: **Explanation:** The correct answer is **Sodium (A)**. Plasma osmolality is primarily determined by the concentration of solutes that are restricted to the extracellular fluid (ECF) compartment. **Why Sodium is the correct answer:** Sodium is the most abundant cation in the ECF. According to the formula for calculating estimated plasma osmolality: **Calculated Osmolality = 2 × [Na⁺] + [Glucose]/18 + [BUN]/2.8** Since the normal concentration of Sodium is approximately 135–145 mEq/L, doubling it accounts for nearly 270–290 mOsm/kg of the total normal plasma osmolality (which is roughly 285–295 mOsm/kg). Sodium, along with its associated anions (Chloride and Bicarbonate), contributes over 90% of the total plasma osmotic pressure. **Why other options are incorrect:** * **Potassium (B):** While Potassium is the major determinant of **intracellular** osmolality, its plasma concentration is very low (3.5–5.0 mEq/L), making its contribution to plasma osmolality negligible. * **Glucose (C):** Under normal physiological conditions, glucose contributes only about 5–6 mOsm/kg. It only becomes a significant contributor in pathological states like Diabetes Mellitus (Hyperglycemic Hyperosmolar State). * **Calcium (D):** Calcium exists in very small concentrations in the plasma (~9–11 mg/dL or ~2.5 mmol/L) and does not significantly impact total osmolality. **Clinical Pearls for NEET-PG:** * **Osmolar Gap:** The difference between measured osmolality and calculated osmolality. A gap >10 mOsm/kg suggests the presence of unmeasured osmotically active substances (e.g., Ethanol, Methanol, Ethylene glycol). * **Major Intracellular Cation:** Potassium. * **Major Extracellular Cation:** Sodium. * **Plasma Oncotic Pressure:** Primarily maintained by **Albumin**, not electrolytes.

Question 56: What is the concentration of sodium (mEq/L) in normal saline?

A. 77
B. 109
C. 130
D. 154 (Correct Answer)

Explanation: **Explanation:** Normal Saline (0.9% NaCl) is an isotonic crystalloid solution widely used in clinical practice. The concentration of 154 mEq/L is derived from its chemical composition: 0.9% NaCl means there are 0.9 grams of Sodium Chloride per 100 mL of solution, which equals **9 grams per Liter**. To calculate the milliequivalents: * Molecular weight of NaCl ≈ 58.5 g/mol. * 9g / 58.5 ≈ 0.154 mol/L. * Since NaCl dissociates into Na⁺ and Cl⁻, there are **154 mEq/L of Sodium** and **154 mEq/L of Chloride**. **Analysis of Options:** * **Option A (77 mEq/L):** This is the sodium concentration found in **Half-Normal Saline (0.45% NaCl)**, often used as a maintenance fluid to provide free water. * **Option B (109 mEq/L):** This is the concentration of **Chloride** (not sodium) in **Ringer’s Lactate**. * **Option C (130 mEq/L):** This is the concentration of **Sodium** in **Ringer’s Lactate**, making it more physiological than Normal Saline. * **Option D (154 mEq/L):** Correct. It represents the standard sodium content in 0.9% NaCl. **High-Yield Clinical Pearls for NEET-PG:** 1. **Osmolarity:** The theoretical osmolarity of Normal Saline is **308 mOsm/L** (154 Na + 154 Cl). 2. **Hyperchloremic Metabolic Acidosis:** Large volumes of Normal Saline can lead to this condition due to the high chloride content (154 mEq/L) compared to plasma (approx. 100 mEq/L). 3. **Isotonicity:** While called "Normal," it is slightly hypertonic to plasma (normal plasma osmolarity is ~285–295 mOsm/L).

Question 57: What is the major cation in the intracellular space?

A. Proteins
B. Potassium (Correct Answer)
C. Sodium
D. Chloride

Explanation: **Explanation:** The distribution of electrolytes across cell membranes is fundamental to cellular physiology. The body maintains a distinct chemical gradient between the **Intracellular Fluid (ICF)** and **Extracellular Fluid (ECF)**. **Why Potassium is Correct:** Potassium ($K^+$) is the **primary intracellular cation**. Approximately 98% of the body's total potassium is located inside the cells, with an intracellular concentration of about **140-150 mEq/L**, compared to only 3.5-5.0 mEq/L in the ECF. This gradient is primarily maintained by the **$Na^+$-$K^+$ ATPase pump**, which actively pumps three $Na^+$ ions out of the cell and two $K^+$ ions into the cell. **Analysis of Incorrect Options:** * **Sodium ($Na^+$):** This is the **major extracellular cation**. It is crucial for maintaining ECF volume and osmotic pressure. * **Chloride ($Cl^-$):** This is the **major extracellular anion**. It typically follows sodium to maintain electrical neutrality. * **Proteins:** While proteins are the **major intracellular anions** (along with organic phosphates), they are not cations (positively charged ions). **High-Yield Clinical Pearls for NEET-PG:** * **Major Intracellular Cation:** Potassium ($K^+$) * **Major Intracellular Anion:** Phosphates and Proteins * **Major Extracellular Cation:** Sodium ($Na^+$) * **Major Extracellular Anion:** Chloride ($Cl^-$) * **Gibbs-Donnan Effect:** Explains the distribution of diffusible ions in the presence of non-diffusible proteins. * **Clinical Correlation:** Insulin and Alkalosis cause an intracellular shift of $K^+$, leading to hypokalemia. Conversely, cell lysis (e.g., Tumor Lysis Syndrome) releases $K^+$ into the ECF, causing hyperkalemia.

Question 58: Which of the following is also called the "sucrose space"?

A. Plasma membrane
B. Intracellular fluid (ICF)
C. Extracellular fluid (ECF) (Correct Answer)
D. Cerebrospinal fluid (CSF)

Explanation: **Explanation:** The term **"Sucrose Space"** refers to the **Extracellular Fluid (ECF)** volume. This nomenclature is derived from the indicator dilution method used to measure body fluid compartments. **Why ECF is the correct answer:** To measure a specific fluid compartment, we use a marker substance that distributes uniformly within that compartment but does not cross into others. **Sucrose** is a large, polar molecule that can freely pass through capillary endothelium into the interstitial space but **cannot cross the cell membrane** to enter the intracellular compartment. Therefore, when a known amount of sucrose is injected, its volume of distribution represents the entire ECF (Plasma + Interstitial fluid). Other markers for ECF include Inulin, Mannitol, and Sodium isotopes. **Analysis of Incorrect Options:** * **Plasma membrane:** This is a physical lipid bilayer barrier, not a fluid "space" or compartment measured by indicators. * **Intracellular fluid (ICF):** There is no direct marker for ICF because no substance distributes exclusively inside cells without passing through the ECF. ICF is calculated indirectly: *Total Body Water (TBW) – ECF volume*. * **Cerebrospinal fluid (CSF):** This is a component of transcellular fluid (a sub-fraction of ECF). It is too small to be represented by the sucrose distribution. **High-Yield Facts for NEET-PG:** * **TBW Markers:** Deuterium oxide ($D_2O$), Tritiated water ($THO$), and Aminopyrine. * **Plasma Volume Markers:** Evans Blue (T-1824) and Radio-iodinated Albumin (RISA). * **The 60-40-20 Rule:** TBW is 60% of body weight; ICF is 40%; ECF is 20%. * **Inulin** is considered the "Gold Standard" for measuring ECF, though sucrose is a common alternative.

Question 59: Hyponatremia results in?

A. Increased excretion of potassium
B. Increased secretion of aldosterone (Correct Answer)
C. Increased secretion of ADH
D. All of the above

Explanation: **Explanation:** The regulation of serum sodium and fluid balance is primarily governed by the **Renin-Angiotensin-Aldosterone System (RAAS)**. **Why Option B is Correct:** Hyponatremia (low serum sodium) is a potent stimulus for the release of **Aldosterone** from the zona glomerulosa of the adrenal cortex. This occurs via two pathways: 1. **Direct Stimulation:** Low sodium levels directly stimulate the adrenal cortex. 2. **Indirect Stimulation (RAAS):** Hyponatremia often correlates with decreased effective circulating volume, which triggers Renin release from the juxtaglomerular apparatus. Renin converts Angiotensinogen to Angiotensin I, which is then converted to Angiotensin II, the primary stimulator for Aldosterone secretion. Aldosterone acts on the distal convoluted tubule and collecting ducts to increase sodium reabsorption (and water follows), thereby correcting the hyponatremia. **Why Other Options are Incorrect:** * **Option A:** Aldosterone causes sodium reabsorption in exchange for potassium and hydrogen ion secretion. Therefore, while hyponatremia leads to aldosterone secretion, the *direct* result of hyponatremia is not increased potassium excretion; rather, it is the compensatory mechanism to save sodium. * **Option C:** ADH (Vasopressin) is primarily regulated by **plasma osmolarity**. In hyponatremia (low osmolarity), ADH secretion is typically **inhibited** to allow for the excretion of free water, which helps raise serum sodium levels. Increased ADH would worsen hyponatremia by diluting the blood further. **High-Yield Clinical Pearls for NEET-PG:** * **Most common electrolyte abnormality** in hospitalized patients: Hyponatremia. * **Primary stimulus for Aldosterone:** Hyperkalemia (most potent) and Angiotensin II (due to hyponatremia/hypovolemia). * **Pseudohyponatremia:** Seen in severe hyperlipidemia or hyperproteinemia (normal serum osmolality). * **Correction Caution:** Rapid correction of chronic hyponatremia can lead to **Osmotic Demyelination Syndrome** (Central Pontine Myelinolysis).

Question 60: Compared to plasma, the cerebrospinal fluid has a higher concentration of which of the following?

A. Calcium (Ca++)
B. Sodium (Na+)
C. Chloride (Correct Answer)
D. Glucose

Explanation: **Explanation:** The composition of Cerebrospinal Fluid (CSF) is strictly regulated by the blood-CSF barrier (choroid plexus). While CSF is an ultrafiltrate of plasma, it is not identical to it; it is produced via active transport mechanisms that create specific concentration gradients. **1. Why Chloride is Correct:** CSF is essentially a "high-chloride, low-protein" fluid compared to plasma. To maintain electrical neutrality across the blood-brain barrier, the lower concentration of negatively charged proteins and bicarbonate in the CSF is compensated for by a **higher concentration of Chloride (Cl⁻)**. * **Plasma Cl⁻:** ~100–105 mEq/L * **CSF Cl⁻:** ~120–125 mEq/L **2. Why the other options are incorrect:** * **Calcium (Ca²⁺):** The concentration of calcium in the CSF is significantly lower (about 50%) than in the plasma because only the ionized fraction crosses the barrier, and active transport limits its entry. * **Sodium (Na⁺):** Sodium levels are approximately **equal** in both plasma and CSF (~140–145 mEq/L). It does not have a "higher" concentration in CSF. * **Glucose:** CSF glucose is typically **60–70% of the plasma glucose** levels (approx. 45–80 mg/dL). A drop in this ratio is a critical marker for bacterial meningitis. **High-Yield Clinical Pearls for NEET-PG:** * **Higher in CSF:** Chloride, Magnesium (Mg²⁺), and Hydrogen ions (making CSF slightly more acidic than plasma, pH ~7.33). * **Lower in CSF:** Protein (very low: 15–45 mg/dL), Glucose, Calcium, Potassium (K⁺), and Bicarbonate. * **Equal:** Osmolarity and Sodium. * **Diagnostic Tip:** In pyogenic (bacterial) meningitis, CSF chloride and glucose levels **decrease**, while protein levels **increase**.

Practice by Chapter

Body Fluid Compartments and Composition

Practice Questions

Osmolality and Tonicity

Practice Questions

Sodium and Water Balance

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Potassium Homeostasis

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Calcium and Phosphate Regulation

Practice Questions

Magnesium Metabolism

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Fluid Shifts Between Compartments

Practice Questions

Edema Formation Mechanisms

Practice Questions

Dehydration Physiology

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Disorders of Electrolyte Balance

Practice Questions

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