Electrolytes and Body Fluids — MCQs
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A patient with mild congestive heart failure is treated with high-dose furosemide and diuresis of fluid. A complete blood count (CBC) taken before the diuresis shows an RBC count of 4 million/mm3; a CBC taken after diuresis shows a RBC count of 7 million/mm3. Which of the following is the most likely explanation?
Hypokalemia is likely to be seen in which of the following conditions?
What characterizes the sick cell syndrome?
Hypernatremia causes all except?
What is the normal serum potassium level threshold for defining hypokalemia?
Hyperkalemia can occur in all the following conditions EXCEPT?
Which of the following conditions is associated with an increase in blood magnesium levels?
Which of the following causes filtration at the arteriolar end of the capillary bed?
What is the approximate water content in an infant?
What is the percentage of Na+ in 0.9% NaCl solution?
Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs
Question 91: A patient with mild congestive heart failure is treated with high-dose furosemide and diuresis of fluid. A complete blood count (CBC) taken before the diuresis shows an RBC count of 4 million/mm3; a CBC taken after diuresis shows a RBC count of 7 million/mm3. Which of the following is the most likely explanation?
- A. Cyanotic heart disease
- B. Increased erythropoietin
- C. Polycythemia vera
- D. Relative polycythemia (Correct Answer)
Explanation: ### Explanation **Correct Option: D. Relative polycythemia** The core concept here is the distinction between **absolute** and **relative** changes in blood cell concentration. 1. **Why it is correct:** Polycythemia refers to an increase in the concentration of Red Blood Cells (RBCs). In this clinical scenario, the patient received high-dose furosemide (a potent loop diuretic), leading to significant fluid loss (diuresis). This reduces the **plasma volume** while the total mass of RBCs remains unchanged. Because RBC count is measured as cells per unit volume, the contraction of the extracellular fluid (ECF) leads to **hemoconcentration**. This is termed "Relative Polycythemia" because the increase is due to decreased plasma volume, not increased erythropoiesis. **Why the other options are incorrect:** * **A & B (Cyanotic heart disease / Increased EPO):** These cause **Secondary Absolute Polycythemia**. Chronic hypoxia triggers the kidneys to release Erythropoietin (EPO), stimulating the bone marrow to produce *more* RBCs. This process takes weeks; it cannot happen acutely following a dose of diuretics. * **C (Polycythemia vera):** This is a **Primary Absolute Polycythemia**, a myeloproliferative neoplasm where the bone marrow produces excess RBCs independent of EPO levels. It is a chronic condition and would not be triggered by diuresis. --- ### High-Yield Clinical Pearls for NEET-PG * **Gaisböck Syndrome:** A classic presentation of relative polycythemia often seen in stressed, hypertensive, middle-aged men who are overweight (also called "Stress Polycythemia"). * **Hematocrit (Hct) vs. Plasma Volume:** If Hct increases but total RBC mass is normal, the diagnosis is always relative polycythemia. * **Furosemide Effect:** Rapid diuresis can lead to "contraction alkalosis" and hemoconcentration, affecting lab values like Hct, Albumin, and BUN. * **Formula:** $\text{Concentration} = \frac{\text{Amount (RBC Mass)}}{\text{Volume (Plasma)}}$. If Volume ↓, Concentration ↑.
Question 92: Hypokalemia is likely to be seen in which of the following conditions?
- A. Insulin therapy (Correct Answer)
- B. Addison's disease
- C. Starvation ketosis
- D. Hemolytic anemias
Explanation: **Explanation:** **Correct Option: A. Insulin therapy** Insulin is a potent stimulator of the **Na⁺-K⁺ ATPase pump** located on cell membranes (primarily in skeletal muscle and liver). When insulin levels rise, it promotes the rapid shift of potassium from the extracellular fluid (ECF) into the intracellular fluid (ICF). This redistribution causes a decrease in serum potassium levels, leading to **hypokalemia**. This is why insulin (along with glucose) is used therapeutically to treat hyperkalemia. **Incorrect Options:** * **B. Addison’s Disease:** This is primary adrenocortical insufficiency characterized by a deficiency of **aldosterone**. Since aldosterone normally promotes K⁺ excretion in the distal tubule, its absence leads to potassium retention and **hyperkalemia**. * **C. Starvation Ketosis:** In states of metabolic acidosis (like ketosis), there is an excess of H⁺ ions in the ECF. To buffer this, H⁺ moves into cells in exchange for K⁺ moving out into the ECF, resulting in **hyperkalemia**. * **D. Hemolytic Anemias:** Potassium is the major intracellular cation. Lysis of red blood cells releases large amounts of intracellular potassium into the plasma, causing **hyperkalemia**. **NEET-PG High-Yield Pearls:** * **Shift Hypokalemia:** Other factors causing an ECF-to-ICF shift include **Beta-2 agonists** (e.g., Salbutamol) and **Alkalosis**. * **ECG in Hypokalemia:** Look for flattened T-waves, **prominent U-waves**, and ST-segment depression. * **Management Tip:** Always check potassium levels before and during the management of Diabetic Ketoacidosis (DKA) with insulin to prevent life-threatening arrhythmias.
Question 93: What characterizes the sick cell syndrome?
- A. Hyponatremia (Correct Answer)
- B. Hypernatremia
- C. Hypokalemia
- D. Hyperkalemia
Explanation: **Explanation:** **Sick Cell Syndrome** refers to a state of generalized cellular dysfunction typically seen in patients with chronic, debilitating illnesses, severe sepsis, or advanced organ failure. **Why Hyponatremia is the Correct Answer:** The hallmark of this syndrome is the **failure of the Na⁺-K⁺ ATPase pump** due to decreased cellular energy (ATP) production or increased membrane permeability. Under normal conditions, this pump maintains high extracellular sodium and high intracellular potassium. When the pump fails: 1. **Sodium enters the cell:** Na⁺ moves down its concentration gradient into the intracellular compartment. 2. **Water follows:** This leads to cellular swelling and a decrease in the concentration of sodium in the extracellular fluid (ECF). 3. The resulting **Hyponatremia** is often "dilutional" or "redistributional" in nature and is frequently resistant to simple saline replacement unless the underlying illness is treated. **Analysis of Incorrect Options:** * **B. Hypernatremia:** This would imply a loss of water or gain of sodium, which is the opposite of the internal redistribution seen in sick cell syndrome. * **C. Hypokalemia:** In sick cell syndrome, as Na⁺ enters the cell, **Potassium (K⁺) leaks out** of the cell into the ECF. Therefore, you are more likely to see a rise in serum potassium rather than a decrease. * **D. Hyperkalemia:** While K⁺ does leak out of cells, the primary diagnostic marker and most characteristic electrolyte abnormality used to define Sick Cell Syndrome in clinical exams is **Hyponatremia**. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Increased cell membrane permeability + Na⁺-K⁺ Pump failure. * **Key Finding:** Asymptomatic hyponatremia in a chronically ill patient. * **Management:** Treatment focuses on the **underlying primary disease** rather than aggressive sodium correction, as the hyponatremia is a marker of severity rather than a primary salt deficit.
Question 94: Hypernatremia causes all except?
- A. Seizure
- B. Thrombus (Correct Answer)
- C. Brain haemorrhage
- D. Central pontine myelinosis
Explanation: ### Explanation **Hypernatremia** is defined as a serum sodium concentration >145 mEq/L. It primarily causes symptoms related to the **Central Nervous System (CNS)** due to the osmotic movement of water out of brain cells. **Why "Thrombus" is the correct answer:** While severe dehydration (often associated with hypernatremia) can lead to hemoconcentration and a theoretical risk of venous stasis, **thrombus formation is not a direct or classic pathological consequence of hypernatremia itself.** In contrast, the other options are well-documented acute neurological complications of rapid shifts in serum sodium. **Analysis of Incorrect Options:** * **Brain Hemorrhage:** As sodium levels rise in the extracellular fluid (ECF), water moves out of the brain cells (osmosis), causing the brain to shrink. This shrinkage puts mechanical tension on the delicate **bridging veins**, leading to their rupture and resulting in intracranial or subarachnoid hemorrhage. * **Seizures:** The rapid dehydration of neurons and associated electrolyte imbalances alter the resting membrane potential, leading to neuronal hyperexcitability and seizures. * **Central Pontine Myelinolysis (CPM):** While CPM (part of Osmotic Demyelination Syndrome) is classically associated with the **rapid correction of hyponatremia**, it can also occur in the setting of severe hypernatremia itself. The osmotic stress leads to the death of oligodendrocytes and subsequent demyelination. **NEET-PG High-Yield Pearls:** * **Brain Shrinkage:** Hypernatremia → Brain cell dehydration → Rupture of bridging veins → Hemorrhage. * **Brain Swelling:** Hyponatremia → Brain cell edema → Herniation. * **Correction Rule:** Never correct chronic hypernatremia faster than **0.5 mEq/L/hr** (or 10-12 mEq/L per day) to avoid cerebral edema. * **Most common cause:** Impaired thirst mechanism or restricted access to water.
Question 95: What is the normal serum potassium level threshold for defining hypokalemia?
- A. Less than 3.5 mEq/L (Correct Answer)
- B. Less than 4.5 mEq/L
- C. Less than 5.6 mEq/L
- D. Less than 6.5 mEq/L
Explanation: ### Explanation **Correct Answer: A. Less than 3.5 mEq/L** Potassium ($K^+$) is the primary intracellular cation, with approximately 98% of the body's potassium located inside cells. The normal range for serum potassium is strictly maintained between **3.5 and 5.0 mEq/L**. **Hypokalemia** is clinically defined as a serum potassium level **less than 3.5 mEq/L**. This threshold is critical because potassium levels significantly influence the resting membrane potential of excitable tissues, particularly the heart and skeletal muscles. **Analysis of Incorrect Options:** * **B (Less than 4.5 mEq/L):** This falls within the normal physiological range (3.5–5.0 mEq/L). While 4.5 is on the higher side of normal, it does not constitute a deficiency. * **C (Less than 5.6 mEq/L):** A level above 5.0 or 5.5 mEq/L is actually the threshold for **hyperkalemia**, the opposite of the condition asked. * **D (Less than 6.5 mEq/L):** This is a dangerously high level. Serum potassium >6.5 mEq/L is classified as severe hyperkalemia, representing a medical emergency due to the risk of cardiac arrest. **High-Yield Clinical Pearls for NEET-PG:** * **ECG Changes in Hypokalemia:** Look for flattened T-waves, **prominent U-waves**, ST-segment depression, and prolonged PR intervals. * **Common Causes:** Loop diuretics (Furosemide), vomiting, diarrhea, and hyperaldosteronism (Conn’s Syndrome). * **Management Tip:** Always check **Magnesium** levels in refractory hypokalemia; low magnesium makes potassium replacement difficult. * **Muscle Effects:** Severe hypokalemia can lead to muscle weakness, paralytic ileus, and even rhabdomyolysis.
Question 96: Hyperkalemia can occur in all the following conditions EXCEPT?
- A. Cushing syndrome (Correct Answer)
- B. Crush injuries
- C. Renal failure
- D. Intravascular hemolysis
Explanation: **Explanation:** The correct answer is **Cushing syndrome** because it is associated with **hypokalemia**, not hyperkalemia. **1. Why Cushing Syndrome is the correct answer:** Cushing syndrome involves an excess of glucocorticoids (cortisol). At high concentrations, cortisol loses its specificity and binds to **Mineralocorticoid Receptors (MR)** in the renal distal tubule. This mimics the action of aldosterone, leading to increased sodium reabsorption and increased **potassium excretion** in the urine. Consequently, patients typically present with hypertension and hypokalemic alkalosis. **2. Why the other options are incorrect (Causes of Hyperkalemia):** * **Crush Injuries:** Massive tissue destruction causes the release of intracellular potassium into the extracellular fluid (ECF), as potassium is the primary intracellular cation. * **Renal Failure:** The kidneys are responsible for 90% of potassium excretion. In renal failure (especially acute kidney injury or end-stage renal disease), the decreased Glomerular Filtration Rate (GFR) leads to potassium retention. * **Intravascular Hemolysis:** Red blood cells are rich in potassium. When they rupture within the circulation, they dump their potassium content directly into the plasma, causing hyperkalemia. **Clinical Pearls for NEET-PG:** * **ECG in Hyperkalemia:** Tall peaked T-waves (earliest sign), flattened P-waves, prolonged PR interval, and widened QRS complex (sine wave pattern). * **Conn’s Syndrome (Primary Hyperaldosteronism):** Another classic cause of hypokalemia due to direct mineralocorticoid excess. * **Pseudohyperkalemia:** Can occur due to hemolysis during blood sampling or severe thrombocytosis/leukocytosis. * **Management:** Calcium gluconate is used for membrane stabilization (cardioprotection), while insulin/glucose and beta-agonists shift K+ intracellularly.
Question 97: Which of the following conditions is associated with an increase in blood magnesium levels?
- A. Uncontrolled Diabetes Mellitus
- B. Liver cirrhosis
- C. Kidney failure (Correct Answer)
- D. Chronic alcoholism
Explanation: **Explanation:** **Correct Answer: C. Kidney failure** The kidneys are the primary regulators of magnesium homeostasis. Approximately 80% of serum magnesium is filtered at the glomerulus, and the majority is reabsorbed in the Thick Ascending Limb (TAL) of the Loop of Henle. In **Kidney Failure** (especially Stage 4 or 5 Chronic Kidney Disease), the glomerular filtration rate (GFR) drops significantly, leading to a decreased excretory capacity. This results in the retention of magnesium, causing **hypermagnesemia**. **Why the other options are incorrect:** * **A. Uncontrolled Diabetes Mellitus:** Glycosuria causes osmotic diuresis, which increases the urinary excretion of magnesium, leading to *hypomagnesemia*. * **B. Liver Cirrhosis:** Often associated with secondary hyperaldosteronism and the use of diuretics, both of which promote magnesium loss. Poor dietary intake also contributes to *hypomagnesemia*. * **D. Chronic Alcoholism:** This is one of the most common causes of *hypomagnesemia* due to ethanol-induced tubular dysfunction (impaired reabsorption) and poor nutritional intake. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Serum Magnesium:** 1.7 to 2.2 mg/dL. * **Hypermagnesemia Signs:** Loss of deep tendon reflexes (earliest sign), respiratory depression, and cardiac arrest (in severe cases). * **Antidote:** **Calcium gluconate** is the physiological antagonist used to treat life-threatening hypermagnesemia. * **Reabsorption Site:** Unlike most electrolytes, the bulk of magnesium (60-70%) is reabsorbed in the **TAL of the Loop of Henle**, not the proximal tubule.
Question 98: Which of the following causes filtration at the arteriolar end of the capillary bed?
- A. Decrease in hydrostatic pressure of capillaries
- B. Increase in hydrostatic pressure of capillaries (Correct Answer)
- C. Increase in oncotic pressure of capillaries
- D. Decrease in oncotic pressure of the interstitium
Explanation: ### Explanation The movement of fluid across a capillary membrane is governed by **Starling’s Forces**, which determine the Net Filtration Pressure (NFP). The formula is: **NFP = (Pc - Pi) - (πc - πi)** #### 1. Why Option B is Correct **Capillary Hydrostatic Pressure (Pc)** is the primary force that "pushes" fluid out of the capillary into the interstitial space. At the **arteriolar end**, the Pc is high (approx. 35 mmHg), which exceeds the opposing oncotic pressure. This high hydrostatic pressure drives **filtration**, allowing water and solutes to move into the tissues. #### 2. Why Other Options are Incorrect * **Option A:** A decrease in hydrostatic pressure would reduce the outward push, favoring fluid retention or reabsorption rather than filtration. * **Option C:** **Capillary Oncotic Pressure (πc)**, exerted by plasma proteins (mainly albumin), is a "pulling" force that keeps fluid inside the vessel. Increasing this would promote **reabsorption**, not filtration. * **Option D:** **Interstitial Oncotic Pressure (πi)** acts to pull fluid out of the capillary. Therefore, a *decrease* in this pressure would reduce the force favoring filtration. #### High-Yield NEET-PG Pearls * **Starling’s Hypothesis:** Filtration occurs at the arteriolar end (Pc > πc), while reabsorption occurs at the venular end (πc > Pc). * **Edema Pathophysiology:** Edema is caused by factors that increase filtration: 1. **Increased Pc:** Heart failure, venous obstruction. 2. **Decreased πc:** Nephrotic syndrome, cirrhosis (hypoalbuminemia). 3. **Increased Capillary Permeability:** Inflammation/Sepsis. 4. **Lymphatic Obstruction:** Elephantiasis or post-mastectomy. * **Albumin** is the single most important protein contributing to the plasma oncotic pressure (approx. 25-28 mmHg).
Question 99: What is the approximate water content in an infant?
- A. 60-70%
- B. 75-80% (Correct Answer)
- C. 80-90%
- D. > 90%
Explanation: **Explanation:** The total body water (TBW) content is inversely proportional to body fat and varies significantly with age and gender. **1. Why Option B is Correct:** In infants, the percentage of TBW is at its highest (excluding the fetus) because they have a relatively low proportion of body fat and a high surface-area-to-mass ratio. At birth, a full-term neonate consists of approximately **75-80% water**. This high percentage is primarily due to a larger extracellular fluid (ECF) compartment compared to adults. **2. Analysis of Incorrect Options:** * **Option A (60-70%):** This range represents the TBW of an **adult male (approx. 60%)** and an older child. As an infant grows, fat content increases and the relative water percentage drops. * **Option C & D (80-90% and >90%):** While a fetus in the early stages of gestation can have a water content of up to 90%, by the time of birth (full-term), it stabilizes at the 75-80% range. Values above 80% are generally physiological only in preterm infants. **Clinical Pearls for NEET-PG:** * **Age Trend:** TBW decreases with age: Fetus (90%) → Full-term Neonate (75-80%) → Adult Male (60%) → Adult Female (50%) → Elderly (45-50%). * **Gender Difference:** Adult females have lower TBW (50%) than males (60%) because they possess a higher percentage of subcutaneous adipose tissue (fat is anhydrous). * **Compartment Shift:** In infants, the **ECF is greater than the ICF**. By age 1, this reverses to the adult pattern where ICF > ECF. This explains why infants are more susceptible to rapid dehydration during diarrheal illnesses.
Question 100: What is the percentage of Na+ in 0.9% NaCl solution?
- A. 0.45% (Correct Answer)
- B. 1.54%
- C. 0.90%
- D. 2.84%
Explanation: ### Explanation **1. Understanding the Correct Answer (Option A: 0.45%)** To calculate the percentage of Sodium (Na+) in a 0.9% NaCl solution, we must look at the molecular weights of the components: * **Molecular Weight of NaCl:** ~58.5 g/mol (Na = 23, Cl = 35.5). * **Proportion of Na+ in NaCl:** $23 / 58.5 \approx 39.3\%$. * **Calculation:** A 0.9% NaCl solution contains 0.9g of NaCl per 100ml. Since Na+ makes up roughly 39.3% of that mass: $0.9 \times 0.393 = 0.354\%$. However, in medical exams, this is often simplified by the ratio of atomic weights. Sodium (23) is approximately **half** the weight of NaCl (58.5). Therefore, the concentration of Na+ is roughly half of 0.9%, which is **0.45%**. **2. Analysis of Incorrect Options** * **Option B (1.54%):** This is a distractor based on the osmolarity. Normal saline has 154 mEq/L of Na+, but this does not translate to 1.54%. * **Option C (0.90%):** This is the concentration of the **entire salt (NaCl)**, not the individual sodium ion. * **Option D (2.84%):** This value is mathematically unrelated to the composition of isotonic saline. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Composition of 0.9% NaCl (Normal Saline):** It contains **154 mEq/L of Na+** and **154 mEq/L of Cl-**. * **Osmolarity:** The theoretical osmolarity is **308 mOsm/L**, making it slightly hypertonic to plasma (normal plasma osmolarity: 275–295 mOsm/L). * **Risk:** Large volumes of 0.9% NaCl can lead to **Hyperchloremic Metabolic Acidosis** due to the high chloride content. * **Isotonicity:** It is considered "isotonic" because its effective osmolality is similar to that of the intracellular fluid, preventing red blood cell lysis.
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
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Edema Formation Mechanisms
Practice Questions
Dehydration Physiology
Practice Questions
Disorders of Electrolyte Balance
Practice Questions
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