Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 101: What percentage of body weight of a one-year-old child is total body water (TBW)?

A. 90%
B. 80%
C. 60% (Correct Answer)
D. 40%

Explanation: ### Explanation The percentage of Total Body Water (TBW) is dynamic and changes significantly during early development due to the reduction of extracellular fluid and the increase in body fat. **1. Why 60% is Correct:** By the age of **one year**, a child’s body composition matures to resemble that of an adult male. At this stage, the TBW stabilizes at approximately **60%** of the total body weight. This transition occurs as the high water content present at birth gradually decreases during the first 12 months of life. **2. Analysis of Incorrect Options:** * **A (90%):** This value is physiologically impossible in a living human. Even a fetus at early gestation has a TBW of about 85-90%, but this drops before birth. * **B (80%):** This is closer to the TBW of a **preterm neonate** (approx. 80%) or a **full-term newborn** (approx. 70-75%). It is too high for a one-year-old. * **C (40%):** This is significantly lower than normal. This value is more representative of the **Intracellular Fluid (ICF)** compartment in an adult (which is 2/3 of TBW, i.e., 40% of body weight), not the Total Body Water. **3. NEET-PG High-Yield Facts:** * **Newborn (Full-term):** TBW is ~75%. * **Infant (1 year):** TBW is ~60% (Adult male levels). * **Adult Female:** TBW is ~50% (Lower than males due to a higher percentage of subcutaneous adipose tissue, which is hydrophobic). * **Elderly:** TBW decreases further (~45-50%) due to loss of muscle mass (sarcopenia). * **Rule of Thumb:** Fat contains very little water; therefore, an increase in body fat percentage always results in a decrease in TBW percentage.

Question 102: Chronic hypokalemia leads to the development of which of the following?

A. Metabolic acidosis
B. Metabolic alkalosis (Correct Answer)
C. Brugada pattern in ECG
D. Vasopressin-resistant diabetes insipidus

Explanation: **Explanation:** Chronic hypokalemia and **metabolic alkalosis** often coexist in a self-perpetuating cycle. The development of alkalosis in hypokalemia occurs through two primary mechanisms: 1. **Intracellular Shifting:** When extracellular potassium ($K^+$) is low, $K^+$ moves out of the cells to maintain serum levels. To maintain electroneutrality, Hydrogen ions ($H^+$) move from the extracellular fluid into the cells. This loss of $H^+$ from the plasma results in metabolic alkalosis. 2. **Renal Compensation (Paradoxical Aciduria):** In the distal tubule, the $Na^+/K^+$ ATPase pump is affected. To reabsorb $Na^+$, the kidney must secrete either $K^+$ or $H^+$. Since $K^+$ is depleted, the kidney preferentially secretes $H^+$ into the urine. This leads to further loss of acid from the body and the characteristic "paradoxical aciduria." **Analysis of Incorrect Options:** * **A. Metabolic acidosis:** Hypokalemia causes alkalosis, not acidosis. Conversely, hyperkalemia is associated with metabolic acidosis (due to $H^+$ shifting out of cells). * **C. Brugada pattern:** This is a genetic sodium channelopathy. While hypokalemia can cause ECG changes (U waves, T-wave flattening, ST depression), it does not cause the pseudo-right bundle branch block and ST elevation seen in Brugada syndrome. * **D. Vasopressin-resistant DI:** While chronic hypokalemia *can* cause Nephrogenic Diabetes Insipidus (by interfering with ADH action on collecting ducts), **Metabolic Alkalosis** is the more direct and classic metabolic consequence tested in this context. **High-Yield Pearls for NEET-PG:** * **Hypokalemia ECG:** "ST depression, shallow T waves, and prominent U waves." * **The "Rule of H":** Hypokalemia, Hypochloremia, and Hyperaldosteronism all typically lead to Metabolic Alkalosis. * **Potassium & Digoxin:** Hypokalemia increases the risk of Digoxin toxicity because $K^+$ and Digoxin compete for the same binding site on the $Na^+/K^+$ ATPase pump.

Question 103: Which of the following constitutes extracellular fluid?

A. Interstitial fluid
B. Blood plasma
C. Both of the above (Correct Answer)
D. None of the above

Explanation: ### Explanation **Concept Overview** Total Body Water (TBW) is divided into two primary compartments: **Intracellular Fluid (ICF)**, which accounts for approximately 2/3 of TBW, and **Extracellular Fluid (ECF)**, which accounts for the remaining 1/3. The ECF is the "internal environment" of the body that surrounds the cells. **Why the Correct Answer is Right** The Extracellular Fluid (ECF) is not a single uniform space but is further subdivided into two major components: 1. **Interstitial Fluid (ISF):** This is the fluid that bathes the cells outside of the vascular system. It constitutes about **3/4 (75%)** of the ECF. 2. **Blood Plasma:** This is the fluid component of blood contained within the intravascular space. It constitutes about **1/4 (25%)** of the ECF. Since both Interstitial fluid and Blood plasma are located outside the cell membranes, **Option C** is the correct answer. **Analysis of Other Options** * **Option A & B:** While both are components of ECF, selecting one over the other would be incomplete. They are separated by the capillary wall, which is highly permeable to water and electrolytes but not to proteins. * **Option D:** This is incorrect as both A and B are the primary constituents of the extracellular environment. **High-Yield Clinical Pearls for NEET-PG** * **Transcellular Fluid:** A small specialized portion of ECF (1–2 liters) includes CSF, intraocular fluid, synovial fluid, and pleural fluid. * **Indicator Dilution Method:** To measure ECF volume, substances that cross capillaries but not cell membranes are used (e.g., **Inulin**, Mannitol, or Sucrose). * **Ionic Composition:** The ECF is rich in **Sodium (Na⁺)**, Chloride (Cl⁻), and Bicarbonate (HCO₃⁻), whereas the ICF is rich in **Potassium (K⁺)** and Magnesium (Mg²⁺). * **Plasma vs. ISF:** The main difference between the two is the higher **protein concentration** in plasma (Donnan effect).

Question 104: What is the normal ratio of cerebrospinal fluid (CSF) glucose to blood glucose?

A. 1/3
B. 1/2
C. 2/3 (Correct Answer)
D. None of the above

Explanation: **Explanation:** The normal concentration of glucose in the cerebrospinal fluid (CSF) is approximately **60% to 70%** of the simultaneous plasma glucose concentration. This corresponds to a ratio of **2/3**. **1. Why 2/3 is Correct:** Glucose enters the CSF from the blood via **facilitated diffusion** using **GLUT-1 transporters** located in the blood-brain barrier (BBB) and the choroid plexus. Because this is a carrier-mediated process rather than simple diffusion, the CSF levels are lower than plasma levels but remain proportional to them. In a healthy adult with a blood glucose of 100 mg/dL, the CSF glucose would typically be around 60–70 mg/dL. **2. Why Other Options are Incorrect:** * **1/3 (Option A):** This ratio is significantly lower than normal and is typically seen in pathological states like bacterial meningitis or fungal infections. * **1/2 (Option B):** While closer, 0.5 is considered the lower limit of the "gray zone." A ratio below 0.5–0.6 is clinically defined as **hypoglycorrhachia**. **3. Clinical Pearls for NEET-PG:** * **Equilibration Time:** It takes about **30 to 60 minutes** for blood glucose changes to reflect in the CSF. Therefore, blood glucose should ideally be measured 1 hour before the lumbar puncture. * **Bacterial vs. Viral Meningitis:** CSF glucose is **markedly decreased** in bacterial, fungal, and tubercular meningitis (due to bacterial consumption and inhibited transport). However, it remains **normal** in most viral meningitis cases. * **GLUT-1 Deficiency Syndrome:** A rare cause of low CSF glucose (hypoglycorrhachia) in the presence of normal blood glucose, leading to seizures and developmental delay.

Question 105: Hypocalcemia is seen with which of the following conditions?

A. Thyrotoxicosis
B. Hyperparathyroidism
C. Acute pancreatitis (Correct Answer)
D. Addison disease

Explanation: **Explanation:** **Correct Option: C. Acute Pancreatitis** Hypocalcemia is a classic metabolic complication of acute pancreatitis. The primary mechanism is **saponification**: during pancreatic inflammation, released lipases break down peripancreatic fat into free fatty acids. These fatty acids bind with circulating calcium ions to form insoluble calcium soaps (salts) in the retroperitoneum. Additionally, a transient decrease in parathyroid hormone (PTH) secretion or resistance to PTH may contribute. Notably, the degree of hypocalcemia is a prognostic marker used in **Ranson’s Criteria** to assess the severity of pancreatitis. **Analysis of Incorrect Options:** * **A. Thyrotoxicosis:** Excess thyroid hormone increases bone turnover by stimulating osteoclastic activity, which typically leads to **hypercalcemia** (seen in ~15-20% of cases). * **B. Hyperparathyroidism:** Primary hyperparathyroidism is the most common cause of outpatient **hypercalcemia** due to increased PTH, which enhances bone resorption, renal calcium reabsorption, and intestinal absorption. * **C. Addison Disease:** Adrenal insufficiency is associated with **hypercalcemia**. The mechanism involves decreased renal calcium clearance and increased bone resorption due to glucocorticoid deficiency. **High-Yield Facts for NEET-PG:** * **Clinical Signs of Hypocalcemia:** Look for **Chvostek sign** (facial twitching on tapping the facial nerve) and **Trousseau sign** (carpal spasm after BP cuff inflation). * **ECG Finding:** The hallmark of hypocalcemia is **QT interval prolongation**. * **Other causes of Hypocalcemia:** Hypoparathyroidism, Vitamin D deficiency, Chronic Kidney Disease (due to hyperphosphatemia and low 1,25-OH Vit D), and Osteoblastic bony metastases (e.g., prostate cancer).

Question 106: Most abundant ion in intracellular fluid is:

A. Protein
B. Bicarbonate
C. Potassium (Correct Answer)
D. Sodium

Explanation: **Explanation:** The distribution of electrolytes across cell membranes is a fundamental concept in physiology, maintained primarily by the **Na⁺/K⁺-ATPase pump**. This pump actively transports three Sodium ions out of the cell and two Potassium ions into the cell, creating distinct chemical gradients. **Why Potassium is Correct:** Potassium ($K^+$) is the **primary intracellular cation**. Approximately 98% of the body's total potassium is located within the cells. Its high intracellular concentration (around 140-150 mEq/L) is crucial for maintaining resting membrane potential, cell volume, and protein synthesis. **Analysis of Incorrect Options:** * **Sodium ($Na^+$):** This is the most abundant **extracellular** cation. Its concentration inside the cell is very low (approx. 10-14 mEq/L). * **Bicarbonate ($HCO_3^-$):** This is a major extracellular buffer. While present intracellularly, its concentration is significantly lower than that of potassium or phosphate. * **Protein:** While proteins are the most abundant intracellular **anions** (providing the negative charge to balance $K^+$), they are not classified as "ions" in the context of this general electrolyte question. If the question specifically asked for the most abundant intracellular *anion*, the answer would be **Phosphate**, followed by proteins. **NEET-PG High-Yield Pearls:** 1. **Major Intracellular Cation:** Potassium ($K^+$). 2. **Major Intracellular Anion:** Phosphate ($PO_4^{3-}$). 3. **Major Extracellular Cation:** Sodium ($Na^+$). 4. **Major Extracellular Anion:** Chloride ($Cl^-$). 5. **Indicator for ICF Volume:** Tritiated water ($D_2O$) or Aminopyrine (measures total body water, from which ECF is subtracted). 6. **Indicator for ECF Volume:** Inulin (Gold Standard), Mannitol, or Sucrose.

Question 107: What is the normal CSF pressure (lumbar)?

A. 70 - 180 mm H2O (Correct Answer)
B. 50 - 100 mm H2O
C. > 200 mm H2O
D. 150 - 200 mm H2O

Explanation: **Explanation:** The normal Cerebrospinal Fluid (CSF) pressure in a healthy adult, when measured via lumbar puncture in the **lateral recumbent position**, typically ranges from **70 to 180 mm H₂O** (or 5–15 mmHg). This pressure is a reflection of the balance between CSF production by the choroid plexus and its absorption through the arachnoid villi into the dural venous sinuses. **Analysis of Options:** * **Option A (Correct):** 70–180 mm H₂O is the standard physiological range. In some texts, up to 200 mm H₂O is considered the upper limit of normal in relaxed adults. * **Option B:** 50–100 mm H₂O is too narrow and represents the lower end of the spectrum. Pressures consistently below 60 mm H₂O may indicate CSF hypotension (e.g., CSF leak). * **Option C:** > 200 mm H₂O is clinically defined as **Intracranial Hypertension**. This can be seen in conditions like idiopathic intracranial hypertension (IIH), meningitis, or intracranial space-occupying lesions. * **Option D:** 150–200 mm H₂O represents only the high-normal range and excludes the typical baseline values. **High-Yield Clinical Pearls for NEET-PG:** 1. **Positioning:** CSF pressure is significantly higher in the sitting position (approx. 200–300 mm H₂O) due to hydrostatic pressure; hence, diagnostic measurements are standardized to the lateral decubitus position. 2. **Queckenstedt's Test:** Historically used to identify spinal canal obstruction; pressing on jugular veins normally causes a rapid rise in CSF pressure. 3. **Pediatric Values:** Normal CSF pressure in infants is lower, generally ranging from 10 to 100 mm H₂O. 4. **Conversion:** 1.36 mm H₂O ≈ 1 mmHg. (Remember: CSF pressure is usually recorded in mm H₂O, while blood pressure is in mmHg).

Question 108: Which of the following hormones stimulates the uptake of potassium into cells?

A. Insulin (Correct Answer)
B. Glucagon
C. Thyroid-stimulating hormone (TSH)
D. Follicle-stimulating hormone (FSH)

Explanation: **Explanation:** The regulation of plasma potassium ($K^+$) is critical for maintaining resting membrane potential. **Insulin** is one of the most potent physiological stimulators of cellular potassium uptake. **1. Why Insulin is Correct:** Insulin stimulates the **$Na^+$-$K^+$ ATPase pump** located in the cell membranes of skeletal muscle, liver, and adipose tissue. By increasing the activity of this pump, insulin promotes the influx of $K^+$ from the extracellular fluid (ECF) into the intracellular fluid (ICF). This occurs independently of its role in glucose transport, although both processes often happen simultaneously. **2. Why Other Options are Incorrect:** * **Glucagon:** Generally has the opposite effect of insulin; it can cause a shift of $K^+$ out of cells (hyperkalemia), particularly in the liver, though its clinical effect is less pronounced than insulin's. * **TSH & FSH:** These are anterior pituitary hormones involved in thyroid regulation and reproductive functions, respectively. They do not have a direct, significant role in acute potassium homeostasis. **3. Clinical Pearls for NEET-PG:** * **Management of Hyperkalemia:** Because insulin shifts $K^+$ into cells, a combination of **Insulin and Dextrose** (to prevent hypoglycemia) is a standard emergency treatment for severe hyperkalemia. * **Other Factors Increasing $K^+$ Uptake:** Besides insulin, **$\beta_2$-adrenergic agonists** (e.g., Salbutamol) and **Aldosterone** also stimulate the $Na^+$-$K^+$ ATPase pump to lower plasma $K^+$. * **Acid-Base Link:** Alkalosis promotes $K^+$ uptake into cells (hypokalemia), while Acidosis causes $K^+$ to shift out of cells (hyperkalemia).

Question 109: Which of the following statements regarding ions in extracellular and intracellular compartments is NOT true?

A. Chloride (Cl-) is the main anion in interstitial fluid.
B. Sodium (Na+) is the main cation in plasma.
C. Magnesium (Mg2+) is the second major cation present in intracellular fluid after Potassium (K+).
D. Bicarbonate (HCO3-) is mainly present in intracellular fluid. (Correct Answer)

Explanation: ### Explanation The distribution of electrolytes between fluid compartments is governed by the **Gibbs-Donnan effect** and the activity of the **Na+-K+ ATPase pump**. **Why Option D is the Correct Answer (The False Statement):** Bicarbonate ($HCO_3^-$) is a primary buffer system of the **Extracellular Fluid (ECF)**, not the Intracellular Fluid (ICF). Its concentration in the ECF (plasma/interstitial fluid) is approximately **24–28 mEq/L**, whereas its concentration inside the cell is significantly lower (around **8–10 mEq/L**). The main intracellular buffers are proteins and organic phosphates. **Analysis of Incorrect Options (True Statements):** * **Option A:** Chloride ($Cl^-$) is indeed the predominant anion in the interstitial fluid and plasma. It balances the positive charge of Sodium to maintain electroneutrality. * **Option B:** Sodium ($Na^+$) is the principal cation of the ECF (Plasma concentration $\approx$ 142 mEq/L). It is the primary determinant of plasma osmolality. * **Option C:** Potassium ($K^+$) is the most abundant intracellular cation ($\approx$ 140 mEq/L). **Magnesium ($Mg^{2+}$)** is the second most abundant intracellular cation, acting as a vital cofactor for ATP-related enzymatic reactions. **High-Yield NEET-PG Pearls:** 1. **Major Cations:** ECF = Sodium ($Na^+$); ICF = Potassium ($K^+$). 2. **Major Anions:** ECF = Chloride ($Cl^-$); ICF = Phosphates ($PO_4^{3-}$) and Proteins. 3. **Calcium:** Most of the body's calcium is in the bone; in the ECF, it is tightly regulated, but its **intracellular** free concentration is extremely low (crucial for cell signaling). 4. **Plasma vs. Interstitial Fluid:** Plasma has a higher protein content than interstitial fluid, leading to a slightly higher cation concentration in plasma due to the Gibbs-Donnan effect.

Question 110: The "sickle cell syndrome" is characterized by what electrolyte imbalance?

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

Explanation: **Explanation:** **Sickle Cell Syndrome** (Sickle Cell Anemia) is frequently associated with **Hyponatremia** due to a combination of renal and physiological factors. The primary mechanism is the impairment of the kidney's concentrating ability, known as **hyposthenuria**. 1. **Why Hyponatremia is Correct:** * **Renal Medullary Infarcts:** Sickling of RBCs in the vasa recta (the low-oxygen, hypertonic environment of the renal medulla) leads to micro-infarctions. This disrupts the countercurrent multiplier system. * **Impaired Water Reabsorption:** The damaged medulla cannot maintain an osmotic gradient, leading to a defect in urinary concentration. * **Compensatory Mechanisms:** Patients often experience chronic volume depletion due to polyuria. This triggers the release of Antidiuretic Hormone (ADH), which causes water retention, further diluting serum sodium levels. Additionally, during a "sickle crisis," increased fluid intake (hydration therapy) can exacerbate dilutional hyponatremia. 2. **Why Incorrect Options are Wrong:** * **Hypernatremia:** This would require significant pure water loss without compensatory intake, which is rare in SCA as the primary defect is a loss of the ability to *conserve* water, usually leading to a steady state of mild volume depletion rather than high sodium. * **Hypokalemia:** Not typically associated with SCA. In fact, the risk is usually in the opposite direction. * **Hyperkalemia:** While hyperkalemia can occur during a crisis due to **hemolysis** (release of intracellular K+) or Type IV Renal Tubular Acidosis (distal tubule damage), **Hyponatremia** is the more classic, chronic electrolyte hallmark mentioned in standard physiological texts regarding the syndrome's effect on total body fluid balance. **High-Yield Clinical Pearls for NEET-PG:** * **Hyposthenuria:** The earliest and most common renal manifestation of Sickle Cell Trait and Disease (inability to concentrate urine > 400-450 mOsm/kg). * **Enuresis:** Common in children with SCA due to the inability to concentrate urine at night. * **Papillary Necrosis:** Can lead to painless gross hematuria in these patients.

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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