Electrolytes and Body Fluids Practice Questions

Q: Transcellular fluids are present in which of the following locations?

Body cavities. **Explanation:** Total Body Water (TBW) is divided into two main compartments: Intracellular Fluid (ICF, 2/3rd) and Extracellular Fluid (ECF, 1/3rd). **Transcellular fluid** is a specialized sub-compartment of the ECF. **Why the correct answer is right:** Transcellular fluids are defined as fluids separated from the main ECF by a layer of epithelium. They are found within **body cavities** and epithelial-lined spaces. Examples include cerebrospinal fluid (CSF), intraocular fluid, synovial fluid, pleural fluid, pericardial fluid, peritoneal fluid, and digestive secretions. Although small in volume (approx. 1–2 liters), they play vital physiological roles. **Why the incorrect options are wrong:** * **Plasma:** This is the fluid component of the blood and is a major sub-compartment of the ECF, but it is not transcellular. * **Interstitial space:** This is the fluid bathing the cells (excluding plasma). It is the largest component of the ECF but is distinct from the transcellular compartment. * **Intranuclear space:** This refers to the fluid inside the nucleus, which is part of the Intracellular Fluid (ICF) compartment. **High-Yield Facts for NEET-PG:** * **Volume:** Transcellular fluid accounts for approximately **1–3%** of TBW. * **Composition:** Unlike plasma or interstitial fluid, its composition is highly variable and specialized (e.g., CSF is low in protein). * **Clinical Pearl:** In certain pathological states (e.g., ascites or pleural effusion), transcellular fluid volume can increase significantly, a phenomenon often referred to as "third-spacing."

Q: Which of the following electrolyte imbalances are seen in a patient with chronic vomiting?

All of the above. Chronic vomiting leads to a complex metabolic state known as **Metabolic Alkalosis with Paradoxical Aciduria**. The development of the electrolyte imbalances mentioned in the options occurs through both direct loss and compensatory mechanisms: 1. **Hypochloremia:** Gastric juice is rich in Hydrochloric acid (HCl). Persistent vomiting leads to the direct loss of chloride ions, making this the primary electrolyte abnormality. 2. **Hyponatremia:** Sodium is lost directly in the vomitus. Furthermore, the resulting volume depletion triggers the release of **ADH (Antidiuretic Hormone)**, which causes water retention, further diluting serum sodium levels. 3. **Hypokalemia:** This occurs via three mechanisms: * Direct loss in gastric juice (minor). * **Secondary Hyperaldosteronism:** Volume depletion activates the RAAS pathway. Aldosterone acts on the distal tubule to reabsorb Na+ and water at the expense of secreting K+ into the urine. * **Bicarbonaturia:** To compensate for alkalosis, the kidney excretes excess HCO3-. Since HCO3- is negatively charged, it "drags" positively charged K+ with it to maintain electrical neutrality. **Why "All of the above" is correct:** Chronic vomiting creates a cycle of volume depletion and acid loss that forces the kidneys to sacrifice sodium, chloride, and potassium to maintain hemodynamic stability and pH balance. **High-Yield Clinical Pearls for NEET-PG:** * **Paradoxical Aciduria:** Despite systemic alkalosis, the urine is acidic because the kidney prioritizes Na+ reabsorption (due to low volume) in exchange for H+ ions when K+ stores are depleted. * **Treatment of Choice:** Isotonic Saline (0.9% NaCl) with Potassium supplementation. Saline corrects the volume deficit and provides chloride, which is essential to stop the renal bicarbonate wasting.

Q: Rapid infusion of insulin causes which of the following?

Hypokalemia. **Explanation:** **Mechanism of Action (Why B is correct):** Insulin is a potent stimulator of the **Na⁺-K⁺ ATPase pump** located on the cell membranes of skeletal muscle and liver cells. When insulin levels rise rapidly, it increases the activity and number of these pumps, leading to an active influx of potassium (K⁺) from the extracellular fluid (ECF) into the intracellular fluid (ICF). This shift results in a rapid decrease in plasma potassium levels, causing **hypokalemia**. **Analysis of Incorrect Options:** * **A. Hyperkalemia:** This is the opposite effect. Hyperkalemia is typically seen in insulin deficiency (e.g., Diabetic Ketoacidosis) because potassium shifts out of the cells. * **C & D. Hypernatremia/Hyponatremia:** While insulin does have some minor effects on renal sodium handling (promoting reabsorption), its primary and most immediate clinical effect during rapid infusion is on potassium distribution. It does not significantly alter serum sodium concentration acutely. **Clinical Pearls for NEET-PG:** 1. **Management of Hyperkalemia:** Because insulin shifts K⁺ into cells, a combination of **Insulin + Dextrose** (to prevent hypoglycemia) is a standard emergency treatment for severe hyperkalemia. 2. **DKA Management:** In Diabetic Ketoacidosis, patients often have high serum K⁺ but low total body K⁺. When treating with insulin, clinicians must monitor K⁺ levels closely, as insulin will drive K⁺ into cells and can precipitate life-threatening hypokalemia. 3. **Other factors shifting K⁺ into cells:** Alkalosis, Beta-2 agonists (e.g., Salbutamol), and Aldosterone.

Q: In a study to detect extracellular fluid volume, 10 gm mannitol was injected intravenously. After adequate time for equilibration of levels, the concentration was measured as 50 mg/100 ml. In this time, 10% of the injected mannitol was excreted. What is the calculated volume of extracellular fluid?

18 Litres. ### Explanation **1. Understanding the Correct Answer (B: 18 Litres)** The volume of a body fluid compartment is calculated using the **Indicator Dilution Principle**: $$Volume = \frac{\text{Amount Injected} - \text{Amount Excreted}}{\text{Final Concentration}}$$ * **Step 1: Calculate Net Amount.** 10 grams were injected, but 10% was excreted. * Excreted = $10\% \text{ of } 10\text{ g} = 1\text{ g}$. * Remaining amount = $10\text{ g} - 1\text{ g} = 9\text{ g}$ (or $9,000\text{ mg}$). * **Step 2: Standardize Units.** The concentration is $50\text{ mg}/100\text{ ml}$, which equals $500\text{ mg/L}$. * **Step 3: Apply Formula.** * $V = \frac{9,000\text{ mg}}{500\text{ mg/L}} = 18\text{ Litres}$. **2. Why Other Options are Incorrect** * **A (10 Litres):** This would be the result if the concentration were $90\text{ mg}/100\text{ ml}$, which is mathematically incorrect based on the data. * **C (42 Litres):** This represents the **Total Body Water (TBW)** in a standard 70 kg male (60% of body weight). Mannitol does not cross cell membranes, so it cannot measure TBW. * **D (52 Litres):** This value is physiologically unlikely for ECF and does not correlate with the calculation provided. **3. Clinical Pearls & High-Yield Facts** * **Indicator Selection:** * **ECF Markers:** Mannitol, Inulin, Sucrose, Sodium ($^{22}\text{Na}$), and Chloride. (Mnemonic: **MISS** - **M**annitol, **I**nulin, **S**ucrose, **S**odium). * **Plasma Volume:** Evans Blue dye or Radio-iodinated Serum Albumin (RISA). * **Total Body Water:** Deuterium oxide ($D_2O$), Tritiated water, or Antipyrine. * **Rule of 60-40-20:** Total Body Water is 60%, ICF is 40%, and ECF is 20% of total body weight. * **Mannitol's Property:** It is the "gold standard" for ECF because it is small enough to pass through capillary pores but too large/polar to cross cell membranes.

Q: If a patient with severe hyperglycemia is given IV insulin, which of the following complications can occur?

Hypokalemia. ### Explanation **Why Hypokalemia is the Correct Answer:** The primary mechanism behind insulin-induced hypokalemia is the activation of the **Na⁺-K⁺ ATPase pump**. Insulin binds to its receptor on skeletal muscle and liver cells, stimulating the pump to move **three Na⁺ ions out** of the cell and **two K⁺ ions into** the cell. This causes a rapid shift of potassium from the extracellular fluid (ECF) into the intracellular fluid (ICF). In patients with severe hyperglycemia (like Diabetic Ketoacidosis), there is often a "pseudo-hyperkalemia" where total body potassium is low due to osmotic diuresis, but serum levels appear normal or high. Administering insulin shifts this remaining serum potassium into cells, potentially leading to life-threatening **hypokalemia**. **Analysis of Incorrect Options:** * **B. Hyperkalemia:** This is incorrect because insulin lowers serum potassium. Hyperkalemia is actually a common finding *before* insulin treatment in acidotic states due to H⁺-K⁺ exchange. * **C. Hyponatremia:** While hyperglycemia itself causes "dilutional hyponatremia" (glucose pulls water into the ECF), insulin therapy corrects this by lowering glucose, which actually helps normalize sodium levels rather than causing further hyponatremia. * **D. Hypernatremia:** Insulin does not directly cause a significant increase in serum sodium. **NEET-PG High-Yield Pearls:** * **Management Rule:** In DKA management, if the patient’s serum potassium is **<3.3 mEq/L**, insulin should be withheld and potassium replacement started first. * **Therapeutic Use:** Because of this shift, a combination of **Insulin + Glucose (G-I drip)** is a standard emergency treatment for severe hyperkalemia. * **Aldosterone vs. Insulin:** Both stimulate the Na⁺-K⁺ ATPase pump, but aldosterone acts primarily on the renal distal tubule, while insulin acts primarily on skeletal muscle.

Q: D2O (heavy water) is used to measure the volume of which body compartment?

Total body water. **Explanation:** The measurement of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = \frac{Amount\ of\ substance}{Concentration}$). To measure a specific compartment, the indicator used must be able to distribute uniformly throughout that compartment and nowhere else. **Why Total Body Water (TBW) is correct:** **D2O (Deuterium oxide)**, also known as heavy water, is an isotope of water. Because it is chemically almost identical to $H_2O$, it freely crosses all cell membranes and capillary walls, distributing evenly across both the extracellular and intracellular compartments. Other markers for TBW include **Tritium ($H_3O$)** and **Antipyrine** (a lipid-soluble drug). **Analysis of Incorrect Options:** * **A. Blood:** Blood volume is measured using **Radio-labeled Albumin** (for plasma) or **Chromium-51 labeled RBCs** (for red cell mass). * **C. Extracellular Fluid (ECF):** Markers for ECF must be able to cross capillaries but *not* cell membranes. Examples include **Inulin** (Gold Standard), **Mannitol**, and **Sucrose**. * **D. Intracellular Fluid (ICF):** There is no direct marker for ICF because no substance distributes *only* inside cells. It is calculated indirectly: $ICF = TBW - ECF$. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of 60-40-20:** TBW is ~60% of body weight, ICF is ~40%, and ECF is ~20%. * **Plasma Volume:** Measured using **Evans Blue dye** or $I^{131}$-Albumin. * **Interstitial Fluid:** Like ICF, it cannot be measured directly. It is calculated as $ECF - Plasma\ Volume$. * **Inulin** is the most accurate marker for ECF because it is not metabolized and is strictly excluded from the intracellular space.

Q: In hypokalaemia, which of the following findings is typically NOT seen?

Increased peristalsis. **Explanation:** In hypokalaemia (serum potassium <3.5 mEq/L), the resting membrane potential of cells becomes more negative (hyperpolarized). This increases the threshold required for action potential generation, leading to **decreased excitability** of both skeletal and smooth muscles. **1. Why "Increased peristalsis" is the correct answer:** Since hypokalaemia reduces the excitability of the smooth muscles in the gastrointestinal tract, it leads to **decreased peristalsis** (hypomotility), not an increase. In severe cases, this can progress to **paralytic ileus**. **2. Analysis of incorrect options:** * **Abdominal distension:** This is a direct consequence of decreased peristalsis. As bowel motility slows down, gas and fluids accumulate in the intestinal lumens, leading to clinical distension. * **Effortless vomiting:** This occurs due to the gastric stasis and reverse peristalsis associated with an adynamic (paralytic) ileus. It is "effortless" because it is not driven by forceful gastric contractions but rather by overflow from a distended, atonic stomach. * **Muscular weakness:** Potassium is crucial for skeletal muscle contraction. Hyperpolarization of the sarcolemma makes it harder for muscles to contract, resulting in weakness that typically ascends from the lower limbs to the trunk and respiratory muscles. **NEET-PG High-Yield Pearls:** * **ECG Changes in Hypokalaemia:** Flattened/inverted T waves, **prominent U waves**, ST-segment depression, and prolonged PR interval (Mnemonic: *"U wave is for hypokalaemia"*). * **Muscle involvement:** It can lead to **Rhabdomyolysis** due to impaired vasodilation in exercising muscles. * **Metabolic association:** Hypokalaemia is frequently associated with **Metabolic Alkalosis** (except in cases of RTA or diarrhea).

Q: All of the following are true about hypernatremia due to diarrhea, except?

Secretory diarrhea. ### Explanation Hypernatremia occurs when there is a deficit of free water relative to sodium. In the context of diarrhea, the mechanism depends on the **tonicity** of the fluid lost. **1. Why "Secretory Diarrhea" is the correct (Except) answer:** Hypernatremia is typically caused by **Osmotic Diarrhea** (e.g., viral gastroenteritis, lactulose use), where the stool fluid is **hypotonic** (contains more water than electrolytes). Losing hypotonic fluid leaves the blood concentrated with sodium. In contrast, **Secretory Diarrhea** (e.g., Cholera, VIPoma) involves the loss of **isotonic** fluid. Since water and sodium are lost in equal proportions, it usually leads to *isovolemic hyponatremia* or normal sodium levels, but not hypernatremia. **2. Analysis of other options:** * **Option A (Serum osmolality >295 mOsm/kg):** Hypernatremia by definition increases plasma tonicity. Since Sodium is the primary determinant of serum osmolality ($2 \times Na + Glucose/18 + BUN/2.8$), elevated sodium will always result in high serum osmolality. * **Option C (Urine osmolality >800 mOsm/kg):** In extrarenal causes of water loss (like diarrhea), the kidneys function normally and attempt to conserve water. This results in maximally concentrated urine (Uosm >800 mOsm/kg). * **Option D (Increased circulating AVP):** High serum osmolality stimulates osmoreceptors in the hypothalamus, leading to the release of Arginine Vasopressin (AVP/ADH) from the posterior pituitary to promote water reabsorption in the collecting ducts. ### NEET-PG High-Yield Pearls: * **Osmotic Gap:** In Osmotic diarrhea, the stool osmotic gap is high (>125 mOsm/kg); in Secretory diarrhea, it is low (<50 mOsm/kg). * **Correction Rate:** Never correct chronic hypernatremia faster than **0.5 mEq/L/hr** (or 10-12 mEq/day) to avoid **Cerebral Edema**. * **Commonest Cause:** The most common cause of hypernatremia in clinical practice is impaired thirst or restricted access to water.

Question 1

Transcellular fluids are present in which of the following locations?

Accepted Answer

Body cavities

Answer

Plasma

Answer

Interstitial space

Answer

Intranuclear space

Question 2

Which of the following electrolyte imbalances are seen in a patient with chronic vomiting?

Accepted Answer

All of the above

Answer

Hyponatremia

Answer

Hypochloremia

Answer

Hypokalemia

Question 3

Rapid infusion of insulin causes which of the following?

Accepted Answer

Hypokalemia

Answer

Hyperkalemia

Answer

Hypernatremia

Answer

Hyponatremia

Question 4

In a study to detect extracellular fluid volume, 10 gm mannitol was injected intravenously. After adequate time for equilibration of levels, the concentration was measured as 50 mg/100 ml. In this time, 10% of the injected mannitol was excreted. What is the calculated volume of extracellular fluid?

Accepted Answer

18 Litres

Answer

10 Litres

Answer

42 Litres

Answer

52 Litres

Question 5

If a patient with severe hyperglycemia is given IV insulin, which of the following complications can occur?

Accepted Answer

Hypokalemia

Answer

Hyperkalemia

Answer

Hyponatremia

Answer

Hypernatremia

Question 6

All of the following are used to measure extracellular fluid (ECF) volume, except?

Accepted Answer

Inulin

Answer

Sucrose

Answer

Sodium chloride

Answer

Heavy water

Question 7

D2O (heavy water) is used to measure the volume of which body compartment?

Accepted Answer

Total body water

Answer

Blood

Answer

Extracellular fluid

Answer

Intracellular fluid

Question 8

In hypokalaemia, which of the following findings is typically NOT seen?

Accepted Answer

Increased peristalsis

Answer

Abdominal distension

Answer

Effortless vomiting

Answer

Muscular weakness

Question 9

All of the following are true about hypernatremia due to diarrhea, except?

Accepted Answer

Secretory diarrhea

Answer

Serum osmolality >295 mOsm/kg

Answer

Urine with osmolality >800 mOsm/kg

Answer

Increased circulating AVP

Question 10

Which of the following is an ineffective osmol?

Accepted Answer

Urea

Answer

Na+

Answer

K+

Answer

All

Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids — MCQs

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