Electrolytes and Body Fluids Practice Questions

Q: Hypernatremia is defined as a plasma sodium concentration greater than what value?

145 mmol/L. **Explanation:** **Hypernatremia** is a common electrolyte disorder defined as a serum or plasma sodium concentration **>145 mmol/L**. Sodium is the primary determinant of plasma osmolality; therefore, hypernatremia almost always indicates a state of **hyperosmolality** and relative water deficit (total body water loss exceeding sodium loss). * **Why Option C is Correct:** The normal physiological range for plasma sodium is tightly regulated between **135–145 mmol/L**. Any value exceeding the upper limit of this reference range (145 mmol/L) is clinically classified as hypernatremia. * **Why Options A & B are Incorrect:** 135 mmol/L is the lower limit of normal; values below this are termed *hyponatremia*. 140 mmol/L represents the mean/average normal sodium level. * **Why Option D is Incorrect:** While 150 mmol/L is a common threshold for "moderate" hypernatremia, it is not the diagnostic cutoff. **NEET-PG High-Yield Pearls:** 1. **Defense Mechanism:** The body’s primary defense against hypernatremia is **thirst** and the release of **Arginine Vasopressin (ADH)**. Hypernatremia rarely occurs in individuals with an intact thirst mechanism and access to water. 2. **Clinical Presentation:** Symptoms are primarily neurological due to brain cell shrinkage (e.g., altered mental status, seizures, intracranial hemorrhage). 3. **Correction Rule:** Rapid correction can lead to **Cerebral Edema**. The goal is to lower sodium by no more than **10–12 mmol/L in 24 hours**. 4. **Diabetes Insipidus:** A classic cause of hypernatremia where there is a failure to concentrate urine (Central or Nephrogenic).

Q: Which of the following is NOT a predominant extracellular ion?

K+. **Explanation:** The distribution of electrolytes across the cell membrane is fundamental to cellular physiology. The body is divided into the **Intracellular Fluid (ICF)** and **Extracellular Fluid (ECF)** compartments, each maintained by selective permeability and active transport mechanisms. **Why K+ is the correct answer:** Potassium (**K+**) is the **predominant intracellular cation**. Approximately 98% of the body's potassium is located inside the cells, maintained by the **Na+-K+ ATPase pump**, which actively pumps 3 Na+ out and 2 K+ into the cell. Its concentration in the ICF is ~140-150 mEq/L, compared to only ~3.5-5.0 mEq/L in the ECF. **Analysis of incorrect options:** * **Na+ (Sodium):** The primary extracellular cation. It is the chief determinant of ECF volume and plasma osmolarity. * **Cl- (Chloride):** The major extracellular anion. It typically follows sodium to maintain electrical neutrality. * **HCO3- (Bicarbonate):** A major extracellular anion and the primary buffer system in the ECF, crucial for maintaining acid-base balance. **NEET-PG High-Yield Pearls:** 1. **Major Cations:** ECF = Sodium (Na+); ICF = Potassium (K+). 2. **Major Anions:** ECF = Chloride (Cl-) and Bicarbonate (HCO3-); ICF = Phosphates and Proteins. 3. **Gibbs-Donnan Effect:** Explains why plasma has a slightly higher protein concentration (and thus different ion distribution) than interstitial fluid. 4. **Clinical Correlation:** Hypokalemia or Hyperkalemia (ECF K+ imbalances) are life-threatening because they drastically alter the resting membrane potential of excitable tissues like the heart.

Q: Cerebrospinal fluid, fluid in the eye, and synovial fluid are examples of which type of fluid?

Transcellular fluid. **Explanation:** The correct answer is **A. Transcellular fluid.** **Why Transcellular Fluid is Correct:** Transcellular fluid is a specialized sub-compartment of the **Extracellular Fluid (ECF)**. It is defined as fluid that is separated from the blood plasma by both a capillary endothelial layer and a layer of **epithelial cells**. These fluids are formed by the active transport activities of cells. Examples include: * **Cerebrospinal fluid (CSF):** Formed by the choroid plexus. * **Intraocular fluid:** Aqueous humor in the eye. * **Synovial fluid:** Found in joint cavities. * **Serous fluids:** Pleural, pericardial, and peritoneal fluids. * **Digestive secretions.** **Why Other Options are Incorrect:** * **B. Extracellular fluid:** While transcellular fluid is technically a part of the ECF, "Transcellular fluid" is the **most specific** and accurate classification for these localized fluids. In NEET-PG, always choose the most specific anatomical/physiological category. * **C. Intracellular fluid (ICF):** This refers to the fluid contained within the cell membranes (approx. 2/3 of total body water). The fluids mentioned in the question exist outside of cells. **NEET-PG High-Yield Pearls:** 1. **Volume:** Transcellular fluid typically accounts for only **1–2%** of total body water (approx. 1–2 Liters). 2. **Composition:** Unlike interstitial fluid, transcellular fluid composition differs significantly from plasma due to the selective barrier of epithelial cells. 3. **Total Body Water (TBW) Rule:** 60-40-20 Rule (60% TBW, 40% ICF, 20% ECF). 4. **Marker for ECF:** Inulin, Mannitol, or Sucrose are used to measure ECF volume.

Q: What is the most common electrolyte disorder encountered in clinical practice?

Hyponatremia. **Explanation:** **Hyponatremia** (defined as serum sodium <135 mEq/L) is the most common electrolyte abnormality encountered in clinical practice, occurring in approximately 15–30% of hospitalized patients. **Why Hyponatremia is the Correct Answer:** The primary reason for its prevalence is the body's complex regulation of water balance via **Antidiuretic Hormone (ADH)**. Many clinical conditions—including heart failure, liver cirrhosis, renal failure, and SIADH—lead to non-osmotic release of ADH or impaired free water excretion. Since sodium is the primary determinant of extracellular fluid osmolality, any disturbance in water homeostasis manifests most frequently as a change in sodium concentration. **Analysis of Incorrect Options:** * **Hypokalemia (D):** While very common (especially in patients on diuretics), it ranks second to hyponatremia in overall clinical frequency. * **Hypernatremia (C):** This is less common because the thirst mechanism is a powerful defense against rising sodium levels. It is typically seen only in infants, the elderly, or comatose patients who cannot access water. * **Hypocalcemia (B):** While common in specific settings like the ICU or post-thyroid surgery, it does not reach the broad epidemiological prevalence of sodium imbalances. **NEET-PG High-Yield Pearls:** * **Most common cause of Hyponatremia:** Iatrogenic (administration of hypotonic IV fluids) and SIADH. * **Clinical Danger:** Rapid correction of chronic hyponatremia can lead to **Osmotic Demyelination Syndrome** (Central Pontine Myelinolysis). Rule of thumb: Do not exceed a correction rate of 8–10 mEq/L in 24 hours. * **Pseudohyponatremia:** Always rule out hyperlipidemia or hyperproteinemia, which can falsely lower sodium readings in older lab methods.

Q: Hypo-osmotic dehydration is seen in which of the following conditions?

Adrenocortical insufficiency. **Explanation:** To solve questions on body fluid compartments, it is essential to analyze the net change in both **solute (Sodium)** and **solvent (Water)**. **1. Why Adrenocortical Insufficiency is correct:** In conditions like Addison’s disease, there is a deficiency of **Aldosterone**. Aldosterone normally functions to reabsorb Sodium ($Na^+$) and excrete Potassium ($K^+$). Its absence leads to "pathological salt wasting." Since more solute (NaCl) is lost relative to water, the ECF becomes **hypo-osmotic**. To maintain osmotic equilibrium, water shifts from the ECF into the ICF, causing cells to swell. This results in **decreased ECF volume** (dehydration) with **low osmolarity**. **2. Analysis of Incorrect Options:** * **Decreased water intake:** This leads to **Hyper-osmotic dehydration**. Pure water loss increases ECF osmolarity, drawing water out of cells. * **Chronic Renal Failure:** This typically presents with **Isosmotic fluid retention** or complex electrolyte imbalances, but not classic hypo-osmotic dehydration. * **SIADH:** This causes **Hypo-osmotic overhydration** (Hyponatremia). Excessive ADH leads to water retention, increasing ECF volume and diluting osmolarity. It is not a dehydration state. **3. NEET-PG High-Yield Pearls:** * **Isosmotic Dehydration:** Seen in diarrhea, vomiting, or hemorrhage (loss of fluid equal to plasma tonicity). * **Hyper-osmotic Dehydration:** Seen in Diabetes Insipidus, excessive sweating, or simple water deprivation. * **The "Shift" Rule:** Water always moves from a compartment of lower osmolarity to higher osmolarity. In hypo-osmotic dehydration, the ICF volume actually *increases* despite overall body fluid loss.

Q: Compared with the intracellular fluid, the extracellular fluid has __________ sodium ion concentration, __________ potassium ion concentration, __________ chloride ion concentration, and __________ phosphate ion concentration.

Higher, lower, higher, lower. ### Explanation The distribution of electrolytes between the **Intracellular Fluid (ICF)** and **Extracellular Fluid (ECF)** is fundamentally governed by the **Na⁺-K⁺ ATPase pump**. This active transport mechanism continuously pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, establishing the primary concentration gradients necessary for cellular excitability. **1. Why Option D is Correct:** * **Sodium (Na⁺):** Primarily an extracellular cation. ECF concentration (~142 mEq/L) is significantly **higher** than ICF (~10-14 mEq/L). * **Potassium (K⁺):** Primarily an intracellular cation. ECF concentration (~4 mEq/L) is significantly **lower** than ICF (~140 mEq/L). * **Chloride (Cl⁻):** The major extracellular anion. ECF concentration (~103 mEq/L) is **higher** than ICF (~4 mEq/L). * **Phosphate (PO₄³⁻):** A major intracellular anion (along with proteins). ECF concentration (~4 mEq/L) is **lower** than ICF (~75-100 mEq/L). **2. Why Other Options are Incorrect:** * **Options A, B, and C** fail because they misidentify the "Primary Cation" rule: Sodium is always high outside; Potassium is always high inside. Any option suggesting low ECF sodium or high ECF potassium/phosphate is physiologically incorrect under normal conditions. **3. NEET-PG High-Yield Clinical Pearls:** * **Osmolality:** Despite different ionic compositions, the total osmolality of ICF and ECF is **equal** (~290–300 mOsm/L) because water moves freely to maintain equilibrium. * **The "Anion Gap" Concept:** In the ECF, the sum of cations (Na⁺) minus the sum of measured anions (Cl⁻ + HCO₃⁻) represents unmeasured anions (like phosphate and albumin). * **Calcium:** Like Sodium, Calcium is significantly **higher** in the ECF (ionized) compared to the free cytosolic concentration in the ICF. * **Magnesium:** It is the **second most abundant intracellular cation** after Potassium.

Q: Each body fluid compartment has different electrolyte concentrations. Which of the following is the most abundant extracellular anion?

Chloride. **Explanation:** The composition of body fluid compartments is a fundamental concept in physiology. The extracellular fluid (ECF) and intracellular fluid (ICF) maintain distinct ionic gradients through the action of selective membrane permeability and active transport pumps (like the Na⁺-K⁺ ATPase). **Why Chloride is Correct:** Chloride (Cl⁻) is the **most abundant extracellular anion**, with a normal plasma concentration of approximately **100–108 mEq/L**. It plays a crucial role in maintaining osmotic pressure and acid-base balance. In the ECF, the high concentration of positive sodium ions is primarily balanced by chloride ions to maintain electrical neutrality. **Analysis of Incorrect Options:** * **B. Bicarbonate (HCO₃⁻):** While it is a major extracellular anion involved in buffering, its concentration is significantly lower (approx. **24–28 mEq/L**) compared to chloride. * **C. Sodium (Na⁺):** Sodium is the most abundant extracellular **cation** (approx. 142 mEq/L), not an anion. * **D. Potassium (K⁺):** Potassium is the most abundant **intracellular cation** (approx. 140 mEq/L). Its extracellular concentration is very low (3.5–5.0 mEq/L). **High-Yield Clinical Pearls for NEET-PG:** * **Most abundant intracellular anion:** Phosphate (followed by proteins). * **Gibbs-Donnan Effect:** Plasma has a slightly higher protein concentration than interstitial fluid; since proteins are negatively charged, plasma chloride levels are slightly lower than interstitial chloride levels to maintain equilibrium. * **Anion Gap:** Calculated as $[Na^+] - ([Cl^-] + [HCO_3^-])$. It represents unmeasured anions (like albumin and phosphates) in the ECF. * **Chloride Shift (Hamburger Phenomenon):** The movement of Cl⁻ into RBCs in exchange for HCO₃⁻ to maintain electrical neutrality during CO₂ transport.

Question 1

Hypernatremia is defined as a plasma sodium concentration greater than what value?

Accepted Answer

145 mmol/L

Answer

135 mmol/L

Answer

140 mmol/L

Answer

150 mmol/L

Question 2

A muscular man weighing 70 kg has a hematocrit of 45%. What would be his plasma volume?

Accepted Answer

2720 ml

Answer

2310 ml

Answer

2695 ml

Answer

2890 ml

Question 3

Which of the following is NOT a predominant extracellular ion?

Accepted Answer

K+

Answer

Na+

Answer

Cl-

Answer

HCO3-

Question 4

Cerebrospinal fluid, fluid in the eye, and synovial fluid are examples of which type of fluid?

Accepted Answer

Transcellular fluid

Answer

Extracellular fluid

Answer

Intracellular fluid

Answer

None of the above

Question 5

What is the most common electrolyte disorder encountered in clinical practice?

Accepted Answer

Hyponatremia

Answer

Hypocalcemia

Answer

Hypernatremia

Answer

Hypokalemia

Question 6

Hypo-osmotic dehydration is seen in which of the following conditions?

Accepted Answer

Adrenocortical insufficiency

Answer

Decreased water intake

Answer

Chronic renal failure

Answer

Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

Question 7

Hypokalemia causes the following except?

Accepted Answer

Paralytic ileus

Answer

Tetany

Answer

Orthostatic hypotension

Answer

Rhabdomyolysis

Question 8

Compared with the intracellular fluid, the extracellular fluid has __________ sodium ion concentration, __________ potassium ion concentration, __________ chloride ion concentration, and __________ phosphate ion concentration.

Accepted Answer

Higher, lower, higher, lower

Answer

Lower, lower, lower, lower

Answer

Higher, lower, lower, higher

Answer

Lower, higher, higher, lower

Question 9

Each body fluid compartment has different electrolyte concentrations. Which of the following is the most abundant extracellular anion?

Accepted Answer

Chloride

Answer

Bicarbonate

Answer

Sodium

Answer

Potassium

Question 10

Which of the following does NOT cause hyperkalemia?

Accepted Answer

Intestinal obstruction

Answer

Crush syndrome

Answer

Hemolysis

Answer

Renal failure

Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids — MCQs

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