Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 11: Which bodily fluid has high potassium and low sodium content?

A. Cerebrospinal fluid (CSF)
B. Perilymph
C. Endolymph (Correct Answer)
D. Pleural fluid

Explanation: **Explanation:** The correct answer is **Endolymph**. In the human body, most extracellular fluids (ECF) resemble plasma, characterized by high sodium ($Na^+$) and low potassium ($K^+$) concentrations. **Endolymph**, found within the membranous labyrinth of the inner ear (scala media), is a unique exception. It is the only ECF that resembles intracellular fluid, containing a **high concentration of $K^+$ (~150 mEq/L)** and a **low concentration of $Na^+$ (~1 mEq/L)**. This composition is maintained by the **stria vascularis**, which actively pumps $K^+$ into the endolymph. This high potassium content is crucial for the depolarization of hair cells during auditory and vestibular transduction. **Analysis of Incorrect Options:** * **A. Cerebrospinal fluid (CSF):** CSF is an ultrafiltrate of plasma. While it has lower protein and slightly different chloride levels than plasma, it remains high in $Na^+$ and low in $K^+$. * **B. Perilymph:** Located in the scala tympani and scala vestibuli, perilymph is chemically similar to CSF and typical ECF (High $Na^+$, Low $K^+$). It bathes the base of the hair cells. * **D. Pleural fluid:** This is a serous fluid that acts as a lubricant in the pleural cavity. Like other interstitial fluids, it follows the standard ECF electrolyte pattern. **Clinical Pearls for NEET-PG:** * **Endocochlear Potential:** The high $K^+$ in endolymph creates a positive potential of **+80 mV**, the highest transepithelial potential in the body. * **Meniere’s Disease:** Caused by **endolymphatic hydrops** (excess endolymph), leading to the triad of vertigo, sensorineural hearing loss, and tinnitus. * **Tip Links:** Mechanical opening of $K^+$ channels at the tips of hair cell stereocilia allows $K^+$ to flow *into* the cell from the endolymph, causing depolarization.

Question 12: Carpopedal spasm with normal serum ionic calcium level is due to:

A. Hypomagnesemia (Correct Answer)
B. Hyponatremia
C. Hypokalemia
D. Acidosis

Explanation: **Explanation:** The correct answer is **Hypomagnesemia**. **Why Hypomagnesemia is correct:** Magnesium (Mg²⁺) and Calcium (Ca²⁺) are both divalent cations that stabilize excitable membranes. Magnesium acts as a natural calcium channel blocker; it competes with calcium for entry into nerve terminals. When magnesium levels are low, there is an increased influx of calcium into the presynaptic nerve terminals, leading to the excessive release of acetylcholine at the neuromuscular junction. This results in neuromuscular hyperexcitability, manifesting as **carpopedal spasm (tetany)**, even if serum ionic calcium levels are within the normal range. **Why the other options are incorrect:** * **Hyponatremia:** Low sodium typically leads to CNS symptoms (cerebral edema, seizures, coma) rather than peripheral tetany. * **Hypokalemia:** Low potassium generally causes muscle **weakness**, paralysis, and U-waves on ECG, rather than spasms. * **Acidosis:** Acidosis actually *increases* the fraction of ionized (active) calcium by decreasing its binding to albumin. It is **Alkalosis** (not acidosis) that causes carpopedal spasm by decreasing ionized calcium levels. **High-Yield Clinical Pearls for NEET-PG:** * **Refractory Hypocalcemia:** If hypocalcemia does not respond to calcium supplementation, always check magnesium levels. Magnesium is required for the secretion and action of Parathyroid Hormone (PTH). * **Trousseau’s and Chvostek’s signs:** These are classic clinical indicators of latent tetany, seen in both hypocalcemia and hypomagnesemia. * **Gitelman Syndrome:** A renal tubular defect often presenting with the triad of hypomagnesemia, hypocalciuria, and metabolic alkalosis.

Question 13: The average value of total body water in a young man is what percentage of body weight?

A. 20-40%
B. 30-50%
C. 60-65% (Correct Answer)
D. 90-96%

Explanation: **Explanation:** The correct answer is **60-65%** (Option C). Total Body Water (TBW) is the sum of all fluids within the body's compartments. In a healthy, young adult male of average build, water constitutes approximately **60% of the total body weight**. This value is slightly lower in females (approx. 50%) due to a higher proportion of subcutaneous adipose tissue, which is hydrophobic and contains very little water. **Breakdown of Options:** * **A & B (20-50%):** These values are too low for a healthy adult. Such percentages are typically seen only in cases of extreme morbid obesity, as fat tissue significantly displaces water content. * **D (90-96%):** This is physiologically impossible for humans. While some aquatic organisms (like jellyfish) have this water content, human tissues require structural proteins and minerals (bone) that account for the remaining 40%. **High-Yield Facts for NEET-PG:** 1. **Age Variation:** TBW is highest in **newborns (approx. 75-80%)** and decreases with age as muscle mass declines and fat content increases. 2. **The 60-40-20 Rule:** * **60%** of body weight is Total Body Water. * **40%** is Intracellular Fluid (ICF). * **20%** is Extracellular Fluid (ECF) (further divided into Interstitial fluid ~15% and Plasma ~5%). 3. **Standard Reference:** For clinical calculations, a "Standard 70 kg Male" is used, where TBW is estimated at **42 Liters**. 4. **Tissue Content:** Muscle tissue is roughly 75% water, while adipose (fat) tissue is only about 10% water. Therefore, lean individuals have a higher TBW percentage than obese individuals.

Question 14: Which of the following statements about shock is true?

A. During dehydration, both intracellular fluid (ICF) and extracellular fluid (ECF) volumes decrease.
B. A fluid loss of 10-20% is compatible with life.
C. Hemorrhage causes intravascular fluid loss.
D. All of the above. (Correct Answer)

Explanation: This question tests the fundamental understanding of fluid compartments and the physiological response to volume depletion, which is critical for managing shock. ### **Explanation of Correct Answer** The correct answer is **D (All of the above)** because each statement accurately describes a physiological aspect of fluid loss and shock: * **Option A (Dehydration):** In dehydration (hypertonic volume contraction), water is lost from the ECF. This increases ECF osmolarity, causing water to move out of the cells via osmosis to maintain equilibrium. Consequently, **both ICF and ECF volumes decrease.** * **Option B (Fluid Loss Tolerance):** The body can compensate for mild to moderate volume loss through baroreceptor reflexes and the RAAS pathway. A loss of **10–20% of total blood volume** (Class I and II shock) is generally compatible with life, as compensatory mechanisms maintain perfusion to vital organs. * **Option C (Hemorrhage):** Hemorrhage is defined as the loss of whole blood from the vascular system. Therefore, the primary and immediate impact is a **decrease in intravascular fluid volume** (a sub-compartment of the ECF). ### **Clinical Pearls for NEET-PG** * **Shock Classification:** Based on the ATLS guidelines, Class I shock involves <15% blood loss (minimal symptoms), while Class IV involves >40% loss (life-threatening). * **Fluid Shifts:** In **Hemorrhage** (Isotonic loss), there is no immediate change in ICF volume because there is no osmotic gradient. In **Dehydration** (Pure water loss), the ICF bears a significant portion of the loss. * **Hematocrit Changes:** In acute hemorrhage, the hematocrit remains normal initially; it only drops after compensatory fluid shifts from the interstitium or IV fluid resuscitation. * **Gold Standard:** The most sensitive indicator of early shock is often an increase in **heart rate (tachycardia)** and **increased serum lactate.**

Question 15: What is the most important factor for maintaining intravascular fluid volume?

A. Hydrostatic pressure in capillaries
B. Osmotic pressure in capillaries (Correct Answer)
C. Hydrostatic pressure in interstitial space
D. Osmotic pressure in interstitial space

Explanation: ### Explanation The distribution of fluid between the intravascular and interstitial compartments is governed by **Starling’s Forces**. **Why Option B is Correct:** The **Capillary Colloidal Osmotic Pressure** (Oncotic Pressure), primarily exerted by plasma proteins like **albumin**, is the most critical factor for maintaining intravascular volume. Since proteins are too large to freely cross the capillary endothelium, they create an osmotic pull that "holds" water inside the blood vessels, opposing the outward movement of fluid. A loss of this pressure (e.g., hypoalbuminemia) leads to fluid leaking into the interstitium, causing edema and intravascular volume depletion. **Analysis of Incorrect Options:** * **Option A (Capillary Hydrostatic Pressure):** This is the pressure exerted by blood against the vessel wall. It promotes **filtration** (movement of fluid *out* of the vessel). While essential for tissue perfusion, its primary role is to move fluid away from the intravascular space, not maintain it. * **Option C (Interstitial Hydrostatic Pressure):** This pressure is usually near zero or slightly negative in most tissues. It opposes filtration but is too weak to be the primary determinant of intravascular volume. * **Option D (Interstitial Osmotic Pressure):** This is exerted by the small amount of protein that leaks into the tissue space. It pulls fluid *out* of the capillaries, thus acting against the maintenance of intravascular volume. **High-Yield Clinical Pearls for NEET-PG:** * **Albumin** contributes to approximately **80%** of the total oncotic pressure of plasma. * **Edema** occurs when: Capillary Hydrostatic Pressure ↑, Plasma Oncotic Pressure ↓, or Lymphatic drainage is blocked. * **Starling Equation:** Net Filtration = $K_f [(P_c - P_i) - \sigma(\pi_c - \pi_i)]$. * In conditions like **Nephrotic Syndrome** or **Liver Cirrhosis**, the decrease in Option B is the direct cause of systemic edema.

Question 16: Hypertonic dehydration would result from which of the following conditions?

A. Gastrointestinal fluid loss
B. Diabetes mellitus (Correct Answer)
C. Primary hypoadrenocorticism
D. Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)

Explanation: **Explanation:** **Hypertonic dehydration** occurs when water loss exceeds solute loss, leading to an increase in extracellular fluid (ECF) osmolarity. This creates an osmotic gradient that draws water out of the cells, causing intracellular dehydration. **Why Diabetes Mellitus is correct:** In uncontrolled Diabetes Mellitus, hyperglycemia leads to **osmotic diuresis**. Glucose acts as an osmotically active particle in the renal tubules, dragging out a disproportionately large volume of free water compared to electrolytes. This results in a net loss of hypotonic fluid, leaving the remaining body fluids hypertonic. **Analysis of Incorrect Options:** * **Gastrointestinal fluid loss (Option A):** Most GI secretions (vomiting/diarrhea) are roughly **isotonic**. Loss of these fluids typically leads to isotonic dehydration (decreased ECF volume with normal osmolarity). * **Primary hypoadrenocorticism (Addison’s Disease) (Option C):** Deficiency of aldosterone leads to excessive sodium loss in urine. This results in **hypotonic dehydration**, where salt loss exceeds water loss, leading to decreased ECF osmolarity. * **SIADH (Option D):** This condition involves excessive water retention due to high ADH levels, leading to **hypotonic overhydration** (hyponatremia), not dehydration. **High-Yield NEET-PG Pearls:** * **Diabetes Insipidus:** Another classic cause of hypertonic dehydration (pure water loss). * **Clinical Sign:** In hypertonic dehydration, the **thirst mechanism** is intensely activated due to cellular dehydration in the hypothalamus. * **Fluid of Choice:** Treatment usually involves hypotonic fluids (e.g., 0.45% Saline or 5% Dextrose) to restore osmolarity gradually. * **Formula:** Plasma Osmolarity $\approx 2[Na^+] + \frac{Glucose}{18} + \frac{BUN}{2.8}$. In DM, the high glucose significantly raises osmolarity.

Question 17: Which one of the following is the major determinant of plasma osmolality?

A. Serum sodium (Correct Answer)
B. Serum potassium
C. Blood glucose
D. Blood urea nitrogen

Explanation: ### Explanation The correct answer is **A. Serum sodium**. **1. Why Serum Sodium is the Major Determinant:** Plasma osmolality is primarily determined by the concentration of solutes in the extracellular fluid (ECF). Sodium is the most abundant cation in the ECF, and because it is always accompanied by anions (mainly chloride and bicarbonate) to maintain electroneutrality, sodium and its associated anions account for approximately **90–95% of the total osmotic pressure** of plasma. The standard formula for calculating **Estimated Plasma Osmolality** is: $$2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$$ Since the concentration of sodium is multiplied by two (to account for anions) and is numerically much higher than glucose or urea, it remains the dominant factor. **2. Why Other Options are Incorrect:** * **B. Serum Potassium:** Potassium is the major **intracellular** cation. While it determines intracellular osmolality, its plasma concentration is very low (3.5–5.0 mEq/L), making its contribution to plasma osmolality negligible. * **C. Blood Glucose:** Under normal physiological conditions, glucose contributes only about 5 mOsm/L. It becomes a significant determinant only in pathological states like Diabetes Mellitus (e.g., HHS). * **D. Blood Urea Nitrogen (BUN):** Urea is an "ineffective osmole" because it freely crosses cell membranes. While it contributes to measured osmolality, it does not create an osmotic gradient across the cell membrane (tonicity). **3. NEET-PG High-Yield Pearls:** * **Normal Plasma Osmolality:** 280–295 mOsm/kg H₂O. * **Osmolar Gap:** The difference between measured osmolality (via osmometer) and calculated osmolality. A gap **>10 mOsm/L** suggests the presence of unmeasured toxins (e.g., Ethanol, Methanol, Ethylene glycol). * **Tonicity vs. Osmolality:** Sodium and Glucose are "effective osmoles" (determine tonicity), whereas Urea is an "ineffective osmole."

Question 18: A 32-year-old man visits for a periodic health examination. He has no complaints. He is 170 cm tall and weighs 75 kg. What are the approximate volumes of total body water, intracellular fluid, and extracellular fluid in this patient, respectively?

A. 40L, 30L, 10L
B. 45L, 30L, 15L (Correct Answer)
C. 45L, 35L, 10L
D. 50L, 25L, 25L

Explanation: To solve this question, we apply the **60-40-20 Rule**, which is the gold standard for calculating body fluid compartments in a healthy adult male. ### 1. Calculation Logic (The "Why") Total Body Water (TBW) is approximately **60% of total body weight**. * **TBW:** 60% of 75 kg = $0.6 \times 75 = \mathbf{45L}$. TBW is further divided into Intracellular Fluid (ICF) and Extracellular Fluid (ECF): * **ICF:** 2/3 of TBW (or 40% of body weight) = $2/3 \times 45 = \mathbf{30L}$. * **ECF:** 1/3 of TBW (or 20% of body weight) = $1/3 \times 45 = \mathbf{15L}$. This matches **Option B** (45L, 30L, 15L). ### 2. Analysis of Incorrect Options * **Option A (40L, 30L, 10L):** Incorrectly assumes TBW is ~53% and uses an incorrect 3:1 ratio for ICF:ECF. * **Option C (45L, 35L, 10L):** While TBW is correct, the distribution is wrong. ICF and ECF must follow the 2:1 ratio. * **Option D (50L, 25L, 25L):** Overestimates TBW (66%) and incorrectly assumes ICF and ECF are equal in volume. ### 3. NEET-PG High-Yield Pearls * **Gender Variations:** In adult females, TBW is lower (**~50%**) due to a higher proportion of subcutaneous fat (fat is hydrophobic and contains little water). * **Age Variations:** Newborns have the highest TBW (**~75-80%**), which decreases with age. * **ECF Sub-compartments:** ECF is composed of Interstitial Fluid (3/4 of ECF) and Plasma (1/4 of ECF). * **Indicator Dilution Method:** Remember the markers used to measure these volumes: * **TBW:** Deuterium oxide ($D_2O$), Tritiated water, or Antipyrine. * **ECF:** Inulin (Gold Standard), Mannitol, or Thiosulfate. * **Plasma:** Evans Blue dye or Radio-iodinated albumin. * **ICF:** Calculated indirectly (TBW minus ECF).

Question 19: Potassium in which compartment is responsible for cardiac and neural function?

A. Intracellular (Correct Answer)
B. Extracellular
C. Intravascular
D. Extravascular

Explanation: **Explanation:** The correct answer is **A. Intracellular**. **Why Intracellular?** Potassium ($K^+$) is the primary intracellular cation, with approximately 98% of the body's total potassium stored inside cells (at a concentration of ~140-150 mEq/L). The **Resting Membrane Potential (RMP)** of excitable tissues, such as cardiac myocytes and neurons, is primarily determined by the ratio of intracellular to extracellular potassium. Because the cell membrane is highly permeable to $K^+$ at rest, the high intracellular concentration allows $K^+$ to leak out, creating the negative electrical charge necessary for cellular excitability. Without this massive intracellular reservoir, the electrochemical gradient required for action potentials in the heart and nerves would collapse. **Why the other options are incorrect:** * **B. Extracellular:** While extracellular $K^+$ levels (3.5–5.0 mEq/L) are clinically monitored and can trigger arrhythmias if abnormal, it is the **intracellular** pool that establishes the gradient and provides the bulk of the ion responsible for maintaining the RMP. * **C & D. Intravascular and Extravascular:** These are sub-compartments of the Extracellular Fluid (ECF). While they are important for transport and homeostasis, they do not represent the primary site where potassium exerts its fundamental physiological role in membrane excitability. **High-Yield Clinical Pearls for NEET-PG:** * **Nernst Equation:** Used to calculate the equilibrium potential of $K^+$, which is roughly -90mV (close to the RMP of -70 to -90mV). * **Insulin & Beta-2 Agonists:** These shift $K^+$ from the ECF into the **Intracellular** compartment by stimulating the $Na^+/K^+$ ATPase pump (used in the emergency management of hyperkalemia). * **Hypokalemia on ECG:** Look for U-waves, T-wave flattening, and ST depression. * **Hyperkalemia on ECG:** Look for Tall tented T-waves, widened QRS, and loss of P-waves.

Question 20: 0.9% sodium chloride solution contains how many mmol/L of sodium?

A. 154 (Correct Answer)
B. 145
C. 131
D. 130

Explanation: **Explanation:** The concentration of sodium in 0.9% Sodium Chloride (Normal Saline) is derived from basic chemical calculations. A 0.9% solution means there are **0.9 grams of NaCl in 100 mL** of water, which equates to **9 grams per Liter (1000 mL)**. To find the millimoles (mmol): 1. **Molecular Weight of NaCl:** Sodium (23) + Chloride (35.5) = 58.5 g/mol. 2. **Calculation:** 9 grams / 58.5 (molecular weight) = 0.1538 mol/L. 3. **Conversion:** 0.1538 mol/L × 1000 = **154 mmol/L**. Since NaCl dissociates completely into Na⁺ and Cl⁻, the solution contains 154 mmol/L of Sodium and 154 mmol/L of Chloride, resulting in an osmolarity of 308 mOsm/L. **Analysis of Incorrect Options:** * **B (145):** This represents the upper limit of the normal physiological range for serum sodium (135–145 mEq/L). Normal saline is actually slightly hypertonic compared to plasma. * **C (131):** This is the sodium concentration found in **Ringer’s Lactate (RL)**, which is more physiological than Normal Saline. * **D (130):** This is the sodium concentration found in **Hartmann's Solution** (a variation of RL). **Clinical Pearls for NEET-PG:** * **Isotonicity:** While 0.9% NaCl is called "Normal" saline, its chloride content (154 mmol/L) is significantly higher than plasma chloride (96–106 mmol/L). * **Complication:** Large volume resuscitation with 0.9% NaCl can lead to **Hyperchloremic Metabolic Acidosis** due to the high chloride load. * **Fluid of Choice:** It is the preferred fluid for patients with **hyponatremia, hypovolemic shock, and metabolic alkalosis** (due to its chloride-rich nature).

Practice by Chapter

Body Fluid Compartments and Composition

Practice Questions

Osmolality and Tonicity

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Homeostasis

Practice Questions

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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