Electrolytes and Body Fluids Practice Questions

Q: A patient develops episodes of vomiting and extreme muscle weakness. What is the most likely cause?

Hypokalemia. **Explanation:** The clinical presentation of vomiting followed by extreme muscle weakness is a classic hallmark of **Hypokalemia** (low serum potassium). **Why Hypokalemia is correct:** Vomiting leads to the loss of gastric HCl, causing metabolic alkalosis. In an attempt to maintain electrical neutrality, the body shifts potassium ions from the extracellular fluid (ECF) into the cells in exchange for hydrogen ions. Furthermore, vomiting causes volume depletion, activating the Renin-Angiotensin-Aldosterone System (RAAS), which increases potassium excretion in the kidneys. Potassium is vital for maintaining the resting membrane potential of excitable tissues. Low levels lead to hyperpolarization of muscle cells, making them less responsive to stimuli, which manifests as **muscle weakness, paralysis, or ileus.** **Why other options are incorrect:** * **Hyponatremia:** Typically presents with neurological symptoms like confusion, seizures, or cerebral edema rather than primary muscle weakness. * **Hypophosphatemia:** While it can cause weakness (due to ATP depletion), it is usually associated with refeeding syndrome or chronic alcoholism, not acute vomiting. * **Hypomagnesemia:** Often co-exists with hypokalemia and can cause tremors or tetany (increased excitability), but isolated vomiting primarily drives potassium loss. **NEET-PG High-Yield Pearls:** * **ECG Findings in Hypokalemia:** Flattened T-waves, prominent **U-waves**, and ST-segment depression. * **Muscle involvement:** Weakness typically starts in the lower extremities and ascends (similar to GBS). * **Metabolic Link:** Hypokalemia is often associated with **Metabolic Alkalosis** and **Paradoxical Aciduria**.

Q: All of the following are used for measurement of ECF volume except?

Antipyrine. **Explanation:** The measurement of body fluid compartments relies on the **Indicator Dilution Principle** ($V = Q/C$). To measure the **Extracellular Fluid (ECF)** volume, a substance must be able to cross capillary walls but **cannot** cross cell membranes. **Why Antipyrine is the Correct Answer:** Antipyrine (along with Deuterium oxide and Tritiated water) is highly lipid-soluble and distributes uniformly across all fluid compartments, including intracellular fluid. Therefore, it is used to measure **Total Body Water (TBW)**, not ECF. Since the question asks for the "except" option, Antipyrine is the correct choice. **Analysis of Incorrect Options (ECF Markers):** * **Inulin:** A polysaccharide that is the "gold standard" for ECF measurement because it does not enter cells and is not metabolized. * **Mannitol:** A sugar alcohol that remains in the ECF and is commonly used in clinical and experimental settings. * **Sodium Thiocyanate:** A crystalloid that distributes throughout the ECF. Other similar markers include radioactive sodium, chloride, and bromide. **High-Yield Clinical Pearls for NEET-PG:** * **Plasma Volume:** Measured using **Evans Blue dye** (T-1824) or Radio-iodinated Serum Albumin (RISA). * **Intracellular Fluid (ICF):** Cannot be measured directly. It is calculated as: $ICF = TBW - ECF$. * **Interstitial Fluid:** Calculated as: $ECF - Plasma\ Volume$. * **Rule of Thumb:** Non-metabolizable saccharides (Inulin, Mannitol) and certain ions are the markers of choice for ECF.

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

Fat woman. **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.

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

Mg. ### 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.

Q: What does 0.9% NaCl solution contain?

154 mEq of chloride in 1000 ml of fluid. ### 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.

Q: What is the approximate osmolarity of human plasma?

290 mOsm/L. **Explanation:** The osmolarity of human plasma is a measure of the concentration of osmotically active particles in a liter of solution. In a healthy adult, the normal range for plasma osmolarity is approximately **280 to 295 mOsm/L**, making **290 mOsm/L** the most accurate representative value among the choices. **Why the correct answer is right:** Plasma osmolarity is primarily determined by electrolytes, specifically **Sodium (Na⁺)** and its associated anions (Chloride and Bicarbonate), which account for nearly 90% of the total osmotic pressure. Glucose and Urea contribute the remaining fraction. The body tightly regulates this value via the hypothalamus-pituitary-adrenal axis (ADH secretion) and the thirst mechanism to maintain cellular volume and homeostasis. **Why the incorrect options are wrong:** * **A (190 mOsm/L):** This value represents extreme hypotonicity. Such low osmolarity would cause massive water shift into cells (cerebral edema), leading to seizures or death. * **C & D (390 & 490 mOsm/L):** These values represent severe hypertonicity. High osmolarity (seen in conditions like Hyperosmolar Hyperglycemic State or severe dehydration) causes cellular shrinkage and neurological dysfunction. **High-Yield NEET-PG Pearls:** 1. **Calculated Osmolarity Formula:** $2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$. 2. **Osmolar Gap:** The difference between measured and calculated osmolarity. A gap **>10 mOsm/L** suggests the presence of unmeasured osmoles (e.g., Ethanol, Methanol, Ethylene glycol). 3. **Osmolality vs. Osmolarity:** While often used interchangeably in clinical practice, *Osmolality* (mOsm/kg) is more accurate as it is independent of temperature and pressure. Normal plasma osmolality is similarly ~285–295 mOsm/kg.

Q: 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

3 < 4 < 1 < 2. ### 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).

Q: Calculate the plasma osmolality of a child with plasma Na+ 125 mEq/L, glucose 108 mg/dL, and BUN 140 mg/dL.

306 mOsm/kg. To calculate plasma osmolality, we use the standard clinical formula which accounts for the primary solutes in the extracellular fluid: Sodium, Glucose, and Blood Urea Nitrogen (BUN). ### **The Formula** **Calculated Plasma Osmolality = 2[Na⁺] + (Glucose / 18) + (BUN / 2.8)** ### **Step-by-Step Calculation** 1. **Sodium component:** 2 × 125 = **250** (Sodium is doubled to account for accompanying anions like Cl⁻ and HCO₃⁻). 2. **Glucose component:** 108 / 18 = **6** (Divided by 18 to convert mg/dL to mmol/L). 3. **BUN component:** 140 / 2.8 = **50** (Divided by 2.8 to convert mg/dL to mmol/L). 4. **Total:** 250 + 6 + 50 = **306 mOsm/kg**. ### **Analysis of Options** * **Option B (306 mOsm/kg) is correct** as it precisely follows the stoichiometric conversion of units. * **Option A (300 mOsm/kg)** is incorrect; it likely misses the full contribution of the significantly elevated BUN. * **Options C and D (312 and 318 mOsm/kg)** are incorrect mathematical results arising from using wrong divisors (e.g., forgetting to divide glucose or BUN by their respective constants). ### **High-Yield Clinical Pearls for NEET-PG** * **Effective Osmolality (Tonicity):** Calculated as **2[Na⁺] + (Glucose / 18)**. Urea is an "ineffective osmole" because it freely crosses cell membranes and does not cause water shifts. * **Osmolar Gap:** The difference between measured osmolality (via osmometer) and calculated osmolality. A gap **>10 mOsm/kg** suggests the presence of unmeasured substances like ethanol, methanol, or ethylene glycol. * **Normal Range:** 275–295 mOsm/kg. In this case, despite hyponatremia (125 mEq/L), the child is hyperosmolar due to severe uremia (BUN 140 mg/dL).

Q: Plasma constitutes what percentage of body weight?

5% of Body Weight. ### 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%).

Question 1

A patient develops episodes of vomiting and extreme muscle weakness. What is the most likely cause?

Accepted Answer

Hypokalemia

Answer

Hyponatremia

Answer

Hypophosphatemia

Answer

Hypomagnesemia

Question 2

All of the following are used for measurement of ECF volume except?

Accepted Answer

Antipyrine

Answer

Inulin

Answer

Mannitol

Answer

Sodium Thiocyanate

Question 3

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

Accepted Answer

Fat woman

Answer

Well-built man

Answer

Well-nourished child

Answer

Fat man

Question 4

Which of the following is an intracellular constituent of the neuromuscular system?

Accepted Answer

Mg

Answer

Ca

Answer

Cl

Answer

Na

Question 5

What does 0.9% NaCl solution contain?

Accepted Answer

154 mEq of chloride in 1000 ml of fluid

Answer

0.9 gm of NaCl in 1000 ml of fluid

Answer

77 mEq of sodium in 1000 ml of fluid

Answer

30 mEq of sodium in 1000 ml of fluid

Question 6

What is the approximate osmolarity of human plasma?

Accepted Answer

290 mOsm/L

Answer

190 mOsm/L

Answer

390 mOsm/L

Answer

490 mOsm/L

Question 7

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

Accepted Answer

Metabolic acidosis

Answer

Hyperkalemia

Answer

Metabolic alkalosis

Answer

Hypernatremia

Question 8

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

Accepted Answer

3 < 4 < 1 < 2

Answer

1 < 2 < 3 < 4

Answer

1 < 3 < 4 < 2

Answer

4 < 3 < 2 < 1

Question 9

Calculate the plasma osmolality of a child with plasma Na+ 125 mEq/L, glucose 108 mg/dL, and BUN 140 mg/dL.

Accepted Answer

306 mOsm/kg

Answer

300 mOsm/kg

Answer

312 mOsm/kg

Answer

318 mOsm/kg

Question 10

Plasma constitutes what percentage of body weight?

Accepted Answer

5% of Body Weight

Answer

20% of Body Weight

Answer

15% of Body Weight

Answer

10% of Body Weight

Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids — MCQs

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