Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 31: Body water content as a percentage of body weight is lowest in which of the following groups?

A. Well-built man
B. Fat woman (Correct Answer)
C. Well-nourished child
D. Fat man

Explanation: **Explanation:** The total body water (TBW) content as a percentage of body weight is inversely proportional to the amount of **adipose tissue (fat)** in the body. Adipose tissue is hydrophobic and contains very little water (approx. 10%), whereas lean muscle mass is highly vascular and contains significantly more water (approx. 75%). **Why "Fat Woman" is the correct answer:** Women generally have a higher percentage of subcutaneous fat and lower muscle mass compared to men due to the influence of estrogen. When a woman is also obese ("fat"), the proportion of adipose tissue increases further, displacing the relative percentage of water. Therefore, an obese woman has the lowest TBW percentage among all groups (approximately 40–45% of body weight). **Analysis of Incorrect Options:** * **A. Well-built man:** Men typically have more lean muscle mass and less fat than women. A "well-built" man has high muscle mass, leading to a high TBW (approx. 60%). * **C. Well-nourished child:** Infants and children have the highest TBW percentages (70–75% in newborns) because they have low fat stores and high surface-area-to-mass ratios. * **D. Fat man:** While an obese man has lower TBW than a lean man, he still typically possesses more muscle mass and less essential fat than an obese woman, making his TBW percentage higher than hers. **NEET-PG High-Yield Pearls:** 1. **Standard TBW Values:** Adult Male ≈ 60%; Adult Female ≈ 50%; Infant ≈ 75%. 2. **Rule of Thumb:** As fat content ↑, TBW % ↓. As age ↑, TBW % ↓. 3. **Fluid Compartments:** TBW is divided into Intracellular Fluid (2/3 or 40% of body weight) and Extracellular Fluid (1/3 or 20% of body weight). 4. **Clinical Significance:** Drug dosing (especially for hydrophilic drugs like aminoglycosides) must be adjusted in obese patients because their volume of distribution is lower relative to their total body weight.

Question 32: ECF volume assessment can be most accurately done by which substance?

A. Sucrose
B. Mannitol
C. Inulin (Correct Answer)
D. Aminopyrine

Explanation: **Explanation:** The assessment of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = \text{Amount} / \text{Concentration}$). To measure the **Extracellular Fluid (ECF)** volume, an ideal marker must be able to cross the capillary endothelium but be unable to cross the cell membrane, ensuring it remains exclusively in the interstitial and intravascular spaces. **Why Inulin is the Correct Answer:** **Inulin** is a polysaccharide that is considered the "gold standard" for measuring ECF volume. It is physiologically inert, not metabolized, and its large molecular size prevents it from entering cells. While other substances like sucrose and mannitol also stay in the ECF, Inulin is preferred in physiological studies for its high accuracy and its simultaneous use in measuring Glomerular Filtration Rate (GFR). **Analysis of Incorrect Options:** * **Sucrose & Mannitol (Options A & B):** These are also used to measure ECF volume. However, they are considered less accurate than Inulin because they can be slightly metabolized or, in the case of mannitol, may enter cells in small amounts under certain metabolic conditions. * **Aminopyrine (Option D):** This is a lipid-soluble marker used to measure **Total Body Water (TBW)** or specifically to estimate intracellular pH and gastric mucosal blood flow. It crosses all cell membranes and thus cannot be used for ECF measurement. **Clinical Pearls for NEET-PG:** * **Total Body Water (TBW):** Measured by Heavy water ($D_2O$), Tritiated water ($HTO$), or Antipyrine. * **Plasma Volume:** Measured by **Evans Blue dye (T-1824)** or Radio-iodinated Serum Albumin (RISA). * **Interstitial Fluid:** Cannot be measured directly; it is calculated as $ECF \text{ volume} - \text{Plasma volume}$. * **Intracellular Fluid (ICF):** Calculated as $TBW - ECF$.

Question 33: Edema is due to which of the following factors?

A. Increased capillary osmotic pressure
B. Decreased lymph flow (Correct Answer)
C. Decreased hydrostatic pressure in capillaries
D. Both increased capillary osmotic pressure and decreased lymph flow

Explanation: **Explanation:** Edema is the accumulation of excess fluid in the interstitial spaces. Its formation is governed by the **Starling Forces**, which determine the movement of fluid between the capillaries and the interstitium. **Why "Decreased lymph flow" is correct:** Under normal physiological conditions, a small amount of fluid and protein constantly leaks out of the capillaries into the interstitium. The **lymphatic system** acts as a "scavenger" mechanism, returning this excess fluid and extravasated proteins back into the circulation. When lymph flow is obstructed or decreased (e.g., in filariasis, post-mastectomy, or congenital lymphedema), fluid and proteins accumulate in the tissue spaces, leading to edema. **Analysis of Incorrect Options:** * **A. Increased capillary osmotic pressure:** This is incorrect because capillary osmotic (oncotic) pressure, exerted primarily by albumin, acts as a "pulling" force that keeps fluid *inside* the vessel. An increase would actually prevent edema. It is **decreased** osmotic pressure (hypoalbuminemia) that causes edema. * **C. Decreased hydrostatic pressure:** Capillary hydrostatic pressure is the "pushing" force that drives fluid out into the tissues. A decrease in this pressure would favor fluid retention within the vessel or reabsorption, thereby reducing edema. **Increased** hydrostatic pressure (e.g., in Heart Failure) causes edema. * **D. Both A and B:** Since Option A is physiologically opposite to the cause of edema, this combination is incorrect. **NEET-PG High-Yield Pearls:** * **Starling’s Equation:** Net Filtration = $K_f [(P_c - P_{if}) - \sigma(\pi_c - \pi_{if})]$. * **Most common cause of localized edema:** Venous obstruction or lymphatic blockage. * **Most common cause of generalized edema:** Congestive Heart Failure (increased $P_c$) or Nephrotic Syndrome (decreased $\pi_c$). * **Myxedema:** A non-pitting edema caused by the accumulation of glycosaminoglycans (hyaluronic acid) in the dermis, typically seen in hypothyroidism.

Question 34: Which of the following is an intracellular constituent of the neuromuscular system?

A. Mg (Correct Answer)
B. Ca
C. Cl
D. Na

Explanation: ### Explanation **Correct Answer: A. Mg (Magnesium)** The distribution of electrolytes across the cell membrane is a fundamental concept in physiology. The body fluids are divided into **Intracellular Fluid (ICF)** and **Extracellular Fluid (ECF)**. **Why Mg is the correct answer:** Magnesium is the **second most abundant intracellular cation** (after Potassium). Within the neuromuscular system, it acts as a vital cofactor for over 300 enzymatic reactions, including those involving ATP. It plays a crucial role in stabilizing membranes and regulating neuromuscular excitability by acting as a natural calcium antagonist. **Analysis of Incorrect Options:** * **B. Ca (Calcium):** While essential for muscle contraction, the concentration of free ionized calcium in the **cytosol is extremely low** ($10^{-7}$ mol/L) compared to the ECF. Most intracellular calcium is sequestered within the sarcoplasmic reticulum, not free in the ICF. * **C. Cl (Chloride):** This is the **primary anion of the ECF**. Its concentration inside the cell is kept low to maintain the resting membrane potential. * **D. Na (Sodium):** This is the **predominant cation of the ECF**. The Na⁺-K⁺ ATPase pump actively extrudes sodium from the cell to maintain this gradient. **NEET-PG High-Yield Pearls:** 1. **Cation Hierarchy:** * **ICF:** Potassium (K⁺) > Magnesium (Mg²⁺) > Sodium (Na⁺). * **ECF:** Sodium (Na⁺) > Potassium (K⁺) > Calcium (Ca²⁺) > Magnesium (Mg²⁺). 2. **Anion Hierarchy:** * **ICF:** Phosphates > Proteins > Bicarbonate > Chloride. * **ECF:** Chloride (Cl⁻) > Bicarbonate (HCO₃⁻). 3. **Clinical Correlation:** Hypomagnesemia often coexists with hypokalemia and hypocalcemia. It leads to neuromuscular irritability (tetany, Chvostek sign) because Mg²⁺ normally inhibits the release of acetylcholine at the neuromuscular junction.

Question 35: What does 0.9% NaCl solution contain?

A. 0.9 gm of NaCl in 1000 ml of fluid
B. 77 mEq of sodium in 1000 ml of fluid
C. 154 mEq of chloride in 1000 ml of fluid (Correct Answer)
D. 30 mEq of sodium in 1000 ml of fluid

Explanation: ### Explanation **Concept of Normal Saline (0.9% NaCl)** The term "0.9% NaCl" refers to a weight/volume concentration, meaning there are **0.9 grams of Sodium Chloride in every 100 ml** of solution. To understand the ionic composition, we calculate the amount in 1000 ml (1 Liter): 1. **Mass:** 0.9 g/100 ml = **9 grams of NaCl per Liter**. 2. **Molarity:** The molecular weight of NaCl is approximately 58.5 g/mol. * $9 \text{ g} \div 58.5 \approx 0.154 \text{ moles/L}$ or **154 mmol/L**. 3. **Electrolytes:** Since NaCl dissociates into one $Na^+$ and one $Cl^-$ ion, 154 mmol of NaCl yields **154 mEq of Sodium** and **154 mEq of Chloride**. **Analysis of Options:** * **Option C (Correct):** As calculated, 0.9% NaCl contains exactly 154 mEq of Chloride per Liter. * **Option A:** Incorrect. It contains 9 grams (not 0.9 g) of NaCl in 1000 ml. * **Option B:** Incorrect. It contains 154 mEq of Sodium, not 77 mEq (77 mEq is found in half-normal saline, 0.45% NaCl). * **Option D:** Incorrect. 30 mEq is significantly lower than the physiological concentration of Normal Saline. **High-Yield Clinical Pearls for NEET-PG:** * **Osmolarity:** The theoretical osmolarity of 0.9% NaCl is **308 mOsm/L** ($154 \text{ Na}^+ + 154 \text{ Cl}^-$). It is considered **isotonic** to plasma (normal plasma osmolarity $\approx 285\text{--}295 \text{ mOsm/L}$). * **Hyperchloremic Metabolic Acidosis:** Infusing large volumes of 0.9% NaCl can lead to this condition because the chloride concentration (154 mEq/L) is much higher than the physiological plasma chloride (approx. 100 mEq/L). * **Fluid of Choice:** It is the preferred fluid for initial resuscitation in hypovolemic shock and the only fluid compatible with blood transfusions.

Question 36: Muscular weakness due to magnesium deficiency is enhanced by the presence of which condition?

A. Hyperkalemia
B. Metabolic alkalosis
C. Metabolic acidosis (Correct Answer)
D. Hypernatremia

Explanation: **Explanation:** The correct answer is **Metabolic acidosis**. **Mechanism:** Magnesium (Mg²⁺) plays a critical role in maintaining the resting membrane potential of nerves and muscles. Magnesium deficiency (hypomagnesemia) leads to neuromuscular irritability and weakness. When **metabolic acidosis** is present, it exacerbates this weakness through two primary mechanisms: 1. **Intracellular Displacement:** In acidosis, excess hydrogen ions (H⁺) enter the cells. To maintain electrical neutrality, potassium (K⁺) and magnesium (Mg²⁺) shift out of the cells into the extracellular fluid, further depleting intracellular stores necessary for muscle contraction. 2. **Ionized Calcium Competition:** Acidosis increases the fraction of ionized calcium in the blood. While this usually stabilizes membranes, the concurrent shift in magnesium levels disrupts the Na⁺/K⁺-ATPase pump and calcium-activated potassium channels, worsening the functional muscle weakness. **Analysis of Incorrect Options:** * **Hyperkalemia (A):** High potassium levels typically cause muscle weakness by depolarizing the resting membrane potential, but it does not specifically enhance the biochemical deficit caused by magnesium deficiency in the same synergistic way as acidosis. * **Metabolic alkalosis (B):** Alkalosis generally promotes the entry of potassium into cells and decreases ionized calcium (leading to tetany), which presents differently than the potentiation of magnesium-related weakness. * **Hypernatremia (D):** Sodium imbalances primarily affect CNS function (osmotic shifts) rather than directly enhancing the peripheral muscular effects of magnesium deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Refractory Hypokalemia:** If a patient’s potassium levels do not normalize with supplementation, always check magnesium levels. Hypomagnesemia promotes renal potassium wasting. * **Gitelman Syndrome:** A classic cause of metabolic alkalosis with hypomagnesemia; however, the *weakness* itself is clinically worsened by *acidotic* states. * **ECG Changes:** Hypomagnesemia mimics hypokalemia (prolonged QT, flattened T waves, and presence of U waves).

Question 37: Arrange the following according to increasing order of their total body water as a percentage of body weight: 1. 6-month-old baby 2. Neonate 3. Young female 4. Young male

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

Explanation: ### Explanation The total body water (TBW) as a percentage of body weight is primarily determined by two factors: **age** and **fat content**. Adipose tissue contains very little water compared to lean muscle mass; therefore, individuals with higher body fat percentages have lower TBW percentages. **1. Why the Correct Answer (C) is Right:** * **Neonate (75-80%):** Newborns have the highest TBW because they have minimal subcutaneous fat and high extracellular fluid volume. * **6-month-old baby (approx. 70%):** As infants grow, they begin to accumulate more fat and their TBW percentage gradually decreases from the neonatal peak. * **Young male (60%):** Adult males have more lean muscle mass and less subcutaneous fat than females, maintaining a higher TBW. * **Young female (50%):** Due to the physiological influence of estrogen, females possess a higher percentage of subcutaneous adipose tissue, resulting in the lowest TBW percentage among these groups. **Increasing Order:** Young female (3) < Young male (4) < 6-month-old (1) < Neonate (2). **2. Why Other Options are Wrong:** * **Options A & B:** These incorrectly place infants as having lower TBW than adults. In physiology, TBW is inversely proportional to age. * **Option D:** This incorrectly suggests that infants have less water than adults and that males have less water than females. **3. High-Yield Clinical Pearls for NEET-PG:** * **Elderly:** TBW decreases further with age (approx. 45-50%) due to loss of muscle mass (sarcopenia). * **Standard Value:** For physiological calculations, the "Standard 70kg Male" is assumed to have a TBW of **60% (42 Liters)**. * **Rule of Thumb:** TBW % = 1/Fat content. More fat = Less water. * **Fluid Compartments:** TBW is divided into Intracellular Fluid (2/3) and Extracellular Fluid (1/3).

Question 38: Which of the following conditions is characterized by these ECG changes?

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

Explanation: ***Hyperkalemia*** - Classic ECG changes include **peaked T waves**, **widened QRS complexes**, and **flattened or absent P waves** as potassium levels rise. - Severe hyperkalemia can progress to a **sine wave pattern** on ECG, which is a life-threatening arrhythmia requiring immediate treatment. *Hyponatremia* - **Sodium imbalances** do not produce characteristic or specific ECG changes. - Clinical manifestations are primarily **neurological** (confusion, seizures) rather than cardiac rhythm disturbances. *Hypernatremia* - Like hyponatremia, **elevated sodium levels** do not cause distinctive ECG abnormalities. - Symptoms are mainly **neurological** (altered mental status, hyperreflexia) due to cellular dehydration. *Hypokalemia* - ECG changes include **flattened T waves**, **prominent U waves**, and **ST depression** - opposite to hyperkalemia findings. - May also cause **prolonged QT interval** and increased risk of **torsades de pointes** arrhythmia.

Question 39: Edema may be caused by any of the following, EXCEPT:

A. An increase in the plasma protein concentration (Correct Answer)
B. An increase in the capillary hydrostatic pressure
C. An increase in the capillary permeability
D. Lymphatic obstruction

Explanation: ### Explanation The movement of fluid between the vascular space and the interstitium is governed by **Starling’s Forces**. Edema occurs when there is an imbalance in these forces, leading to excessive fluid accumulation in the interstitial space. **Why Option A is the Correct Answer:** **Plasma Colloid Osmotic Pressure (πp)**, primarily exerted by albumin, is the force that **draws fluid into** the capillary from the interstitium. An **increase** in plasma protein concentration raises this osmotic pressure, which promotes fluid retention within the vessels and opposes edema formation. Conversely, *hypoalbuminemia* (low protein) is a classic cause of edema. **Why the other options are incorrect (Causes of Edema):** * **B. Increased Capillary Hydrostatic Pressure:** This "pushes" fluid out of the vessels. Common in Heart Failure (systemic edema) or Deep Vein Thrombosis (localized edema). * **C. Increased Capillary Permeability:** When the "sieve" becomes leaky, proteins and fluid escape into the interstitium. This is seen in inflammation, burns, and allergic reactions (Type I hypersensitivity). * **D. Lymphatic Obstruction:** The lymphatic system normally drains the small amount of protein and fluid that leaks into the interstitium. Obstruction (e.g., Filariasis or post-mastectomy) leads to **Lymphedema**. ### High-Yield NEET-PG Pearls: * **Starling’s Equation:** $Net\ Fluid\ Movement = K_f [(P_c - P_i) - \sigma(\pi_p - \pi_i)]$ * **Most important protein** for maintaining oncotic pressure: **Albumin**. * **Myxedema:** A non-pitting edema seen in hypothyroidism caused by the accumulation of glycosaminoglycans (hyaluronic acid) in the dermis, not just fluid imbalance. * **Safety Factors against Edema:** Negative interstitial fluid pressure, high lymph flow, and "washout" of interstitial proteins.

Question 40: Plasma constitutes what percentage of body weight?

A. 20% of Body Weight
B. 15% of Body Weight
C. 10% of Body Weight
D. 5% of Body Weight (Correct Answer)

Explanation: ### Explanation To understand the distribution of body fluids, we apply the **60-40-20 Rule**, which states that Total Body Water (TBW) is approximately 60% of the total body weight. 1. **Total Body Water (60%):** Divided into Intracellular Fluid (ICF) and Extracellular Fluid (ECF). 2. **Intracellular Fluid (40%):** The fluid contained within cells. 3. **Extracellular Fluid (20%):** The fluid outside cells, which is further subdivided: * **Interstitial Fluid:** 15% of body weight (¾ of ECF). * **Plasma:** **5% of body weight** (¼ of ECF). **Why Option D is Correct:** Plasma is the liquid component of blood. Since ECF makes up 20% of body weight and plasma constitutes approximately one-fourth of that ECF, the calculation is $20\% \times 1/4 = 5\%$. Therefore, plasma accounts for roughly 5% of the total body weight. **Analysis of Incorrect Options:** * **Option A (20%):** This represents the total **Extracellular Fluid (ECF)** volume, not just the plasma. * **Option B (15%):** This represents the **Interstitial Fluid** volume (the fluid bathing the cells). * **Option C (10%):** This does not correspond to a standard major fluid compartment in the 60-40-20 model. **High-Yield Clinical Pearls for NEET-PG:** * **Blood Volume:** Total blood volume is approximately **7-8%** of body weight (Plasma 5% + RBC volume 3%). * **Indicator Dilution Method:** * To measure **Plasma Volume**, use **Evans Blue dye** or **Radio-iodinated Albumin** (RISA). * To measure **ECF**, use **Inulin**, Mannitol, or Sucrose. * To measure **TBW**, use **Deuterium oxide ($D_2O$)**, Tritiated water, or Aminopyrine. * **Gender/Age Variations:** TBW is lower in females (approx. 50%) due to higher subcutaneous fat and highest in newborns (approx. 75%).

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

Practice Questions

Fluid Shifts Between Compartments

Practice Questions

Edema Formation Mechanisms

Practice Questions

Dehydration Physiology

Practice Questions

Disorders of Electrolyte Balance

Practice Questions

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