Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 191: What is the typical pH range of intracellular fluid (ICF) compared to extracellular fluid (ECF)?

A. Typically around 7.0, slightly less than ECF (Correct Answer)
B. Typically around 7.4, slightly more than ICF
C. Approximately equal to ECF
D. Significantly higher than ECF

Explanation: ***Typically around 7.0, slightly less than ECF*** - The **intracellular fluid (ICF)** tends to be slightly more acidic due to metabolic processes within cells that produce **acidic byproducts**. - This makes its pH typically around **7.0–7.2**, which is subtly lower than the extracellular fluid. *Typically around 7.4, slightly more than ICF* - A pH of approximately **7.4** is characteristic of **extracellular fluid (ECF)**, which includes plasma and interstitial fluid. - The ECF is maintained within a **narrow, slightly alkaline** range to support cellular function and enzyme activity throughout the body. *Approximately equal to ECF* - While both fluid compartments are maintained within a **narrow physiological range**, their pH values are not exactly equal. - This slight difference is essential for various biological processes, including maintaining **membrane potential** and **enzyme efficiency**. *Significantly higher than ECF* - The ICF pH is **not significantly higher** than ECF; in fact, it is slightly lower. - Maintaining too high a pH intracellularly would disrupt **cellular metabolism** and **protein structure**.

Question 192: Tetany in muscle occurs in spite of normal serum Ca2+ level. Which ion is responsible?

A. Mg2+
B. K+
C. Na+
D. Ionized Ca2+ (Correct Answer)

Explanation: ***Ionized Ca2+*** - While total serum calcium might be normal, **tetany** is specifically caused by a decrease in the concentration of **ionized (free) calcium** in the extracellular fluid. - Ionized calcium is the physiologically active form of calcium responsible for neuromuscular excitability. *Mg2+* - **Hypomagnesemia** can exacerbate hypocalcemia and contribute to tetany, but it is not the primary ion directly responsible for tetany when **total serum calcium is normal**. - A deficiency in Mg2+ can impair the release of **parathyroid hormone** and reduce target organ responsiveness to PTH. *K+* - Abnormalities in **potassium levels** (hypokalemia or hyperkalemia) primarily affect cardiac and muscular excitability, leading to arrhythmias or muscle weakness/paralysis. - While electrolyte imbalances are interconnected, changes in potassium are not the direct cause of tetany due to calcium's role. *Na+* - **Sodium ions** are crucial for nerve impulse transmission and muscle contraction by establishing the resting membrane potential and initiating action potentials. - However, direct changes in sodium concentration do not typically cause tetany; rather, they can lead to neurological symptoms like seizures (hyponatremia) or altered mental status (hypernatremia).

Question 193: At what age do the proportions of intracellular fluid (ICF) and extracellular fluid (ECF) in a child approximate those of an adult?

A. 3 years
B. 4 years
C. 1 year
D. 2 years (Correct Answer)

Explanation: ***2 years*** - By the age of **2 years**, the relative proportions of intracellular fluid (ICF) and extracellular fluid (ECF) in a child reach levels comparable to those found in adults. - Infants have a significantly higher percentage of ECF, which gradually decreases as they grow and mature. - This represents the key transition point where adult fluid compartment ratios are first approximated. *1 year* - At **1 year of age**, the ECF proportion is still relatively higher than in adults, though it has decreased from neonatal levels. - The shift towards adult fluid proportions is ongoing and not yet complete. *3 years* - By **3 years of age**, the fluid proportions are already well-established at adult levels, as this milestone is reached by age 2. - This age comes after the initial approximation point, so it is not the earliest age when adult proportions are reached. *4 years* - At **4 years of age**, the child's fluid distribution is well within adult proportions. - The main transition period for fluid compartment ratios is usually completed by age 2, making this age too late to represent the approximation point.

Question 194: Result of liquorice ingestion

A. Hyperkalemic alkalosis
B. Hypokalemic acidosis
C. Hypernatremic acidosis
D. Hypokalemic alkalosis (Correct Answer)

Explanation: ***Hypokalemic alkalosis*** - **Licorice** contains **glycyrrhizic acid**, which inhibits **11β-hydroxysteroid dehydrogenase** in the kidneys, preventing the conversion of cortisol to inactive cortisone. - This leads to increased cortisol acting on **mineralocorticoid receptors**, mimicking **aldosterone excess**, resulting in **sodium reabsorption**, **potassium excretion** (hypokalemia), and **hydrogen ion excretion** (metabolic alkalosis). *Hyperkalemic alkalosis* - This option is incorrect because licorice ingestion leads to **hypokalemia** due to increased potassium excretion, not hyperkalemia. - While it does cause alkalosis, the associated potassium imbalance is the opposite of this choice. *Hypokalemic acidosis* - This option is incorrect because licorice ingestion causes a **metabolic alkalosis** due to increased hydrogen ion excretion, not acidosis. - Although it correctly identifies hypokalemia, the acid-base disturbance is wrong. *Hypernatremic acidosis* - This option is incorrect as licorice ingestion initially causes **sodium and water retention** (which can lead to hypernatremia in severe cases, but is not the primary driver of the acid-base), but primarily leads to **metabolic alkalosis**, not acidosis. - The combination of hypernatremia and acidosis is not characteristic of licorice toxicity.

Question 195: Plasma volume is measured by ?

A. Inulin
B. Evans blue (Correct Answer)
C. D2O
D. Mannitol

Explanation: ***Evans blue*** - **Evans blue** is a dye that binds to plasma proteins and **does not readily cross capillary membranes**, making it an effective tracer for measuring plasma volume. - After intravenous injection, its concentration can be measured to calculate the dilution space, which corresponds to the **plasma volume**. *Inulin* - **Inulin** is a polysaccharide primarily used to measure the **glomerular filtration rate (GFR)** because it is freely filtered by the glomeruli and neither reabsorbed nor secreted by the renal tubules. - It distributes into the **extracellular fluid compartment** and is not confined to the plasma, making it unsuitable for plasma volume measurement. *Mannitol* - **Mannitol** is an osmotic diuretic that distributes in the **extracellular fluid (ECF)**, it is generally used for its osmotic effects to reduce edema or intracranial pressure. - Due to its distribution beyond the plasma compartment, it is not used directly to measure **plasma volume**. *D20* - **D2O (deuterium oxide)**, or heavy water, is used to measure **total body water (TBW)** as it distributes throughout all fluid compartments of the body. - It does not selectively remain within the plasma compartment, making it unsuitable for measuring **plasma volume** alone.

Question 196: Normal range of serum osmolality is (mOsm/Kg)?

A. 280 - 300 (Correct Answer)
B. 250 - 270
C. 300 - 320
D. 210 - 230

Explanation: ***280 - 300*** - The normal range for **serum osmolality** is generally considered to be 280-300 mOsm/Kg. - This range reflects the appropriate balance of solutes (like sodium, glucose, and urea) in the blood, which is crucial for **fluid homeostasis**. *250 - 270* - A serum osmolality in this range would indicate **hypoosmolality**, suggesting a relative excess of water or a deficit of solutes in the blood. - This could be seen in conditions like **syndrome of inappropriate antidiuretic hormone (SIADH)** or primary polydipsia. *300 - 320* - This range suggests **hyperosmolality**, meaning there's a relative deficit of water or an excess of solutes. - Conditions such as **dehydration**, uncontrolled diabetes mellitus, or severe hypernatremia can lead to values in this range. *210 - 230* - Such a significantly low osmolality would represent severe **hypoosmolality**, usually associated with extreme overhydration or profound solute depletion. - This deviation is critically abnormal and would indicate severe electrolyte imbalance, potentially leading to **cerebral edema**.

Question 197: Which of the following substances has the same concentration in cerebrospinal fluid (CSF) and plasma?

A. Glucose
B. Ca
C. HCO3
D. Cl (Correct Answer)

Explanation: ***Cl*** - **Chloride ions (Cl-)** have the **closest concentration** between CSF and plasma among the listed options, with a CSF-to-plasma ratio of approximately 1.1-1.15. - CSF chloride is **slightly higher** than plasma chloride (CSF: ~120-130 mEq/L; Plasma: ~100-110 mEq/L) because chloride ions freely cross the **blood-brain barrier** and help maintain **electroneutrality** in CSF due to the low protein content. - The elevated chloride compensates for the absence of negatively charged proteins in CSF, making it the **best answer** among the given options. *Glucose* - **Glucose** concentration in CSF is approximately **60-70%** of plasma glucose concentration (CSF: 50-80 mg/dL; Plasma: 70-110 mg/dL). - Transport across the **blood-brain barrier** occurs via **GLUT1 transporters**, which are tightly regulated to meet brain metabolic demands. *Ca* - **Calcium (Ca2+)** concentration in CSF is **significantly lower** than in plasma (CSF: ~2.1-2.5 mg/dL; Plasma: ~8.5-10.5 mg/dL). - Only the **ionized, unbound fraction** can cross the blood-brain barrier, as protein-bound calcium cannot pass through. *HCO3* - **Bicarbonate (HCO3-)** concentration in CSF is typically **slightly lower** than in plasma (CSF: ~20-25 mEq/L; Plasma: ~22-28 mEq/L). - Active regulation maintains **CSF pH** and CO2 buffering capacity independent of plasma bicarbonate levels.

Question 198: Osmolarity is defined as?

A. Number of osmoles per litre (Correct Answer)
B. Number of osmoles per kg
C. Weight of solute per litre of solution
D. Weight of solvent per litre of solution

Explanation: ***Number of osmoles per litre*** - **Osmolarity** is a measure of the **solute concentration** in a solution, specifically the number of **osmoles of solute per liter of solution**. - It is often used in clinical settings to assess the **concentration of dissolved particles** in bodily fluids like plasma. *Number of osmoles per kg* - This definition describes **osmolality**, which measures the concentration of a solution as the **number of osmoles of solute per kilogram of solvent**. - While related, osmolarity and osmolality are distinct terms, with osmolality being less affected by temperature and pressure changes. *Weight of solute per litre of solution* - This definition describes a **mass concentration** (e.g., g/L), but it does not account for the **number of osmotically active particles**. - Different solutes can have the same weight but varying numbers of particles (e.g., 1 mol of glucose vs. 1 mol of NaCl dissociates into 2 particles). *Weight of solvent per litre of solution* - This statement incorrectly relates to solvent quantity rather than solute concentration and is not a standard definition for osmolarity or any related osmotic property. - The focus of osmolarity is on the concentration of the **dissolved particles (solute)**, not the weight of the solvent.

Question 199: What is the normal range of interstitial pressure?

A. -3 to 0 mmHg
B. -5 to 0 mmHg (Correct Answer)
C. 0 to 5 mmHg
D. 5 to 10 mmHg

Explanation: ***-5 to 0 mmHg*** - The interstitial fluid is normally under a **slight negative pressure**, typically ranging from **-5 to 0 mmHg** - This negative pressure helps pull fluid from the capillaries into the interstitial space and facilitates **lymphatic drainage** - Maintained by continuous drainage of fluid and proteins by the **lymphatic system** - This range is the commonly accepted value in standard physiology references for Indian medical exams *-3 to 0 mmHg* - While this range acknowledges the typically **negative nature** of interstitial pressure, it represents a slightly narrower range - Some sources cite this as the average range, but **-5 to 0 mmHg** is the more commonly accepted standard range - Not the most precise or widely cited range for exam purposes *0 to 5 mmHg* - This range suggests a **positive interstitial pressure**, which is generally **abnormal** - Indicates **edema formation** due to excess fluid accumulation in the interstitial space - Positive pressure impairs fluid reabsorption and lymphatic drainage - Represents pathological fluid dynamics *5 to 10 mmHg* - Represents significant **positive interstitial pressure** leading to severe **interstitial edema** - Markedly impairs tissue function and fluid exchange - Indicates pathological conditions where capillary filtration far exceeds lymphatic drainage capacity - Associated with severe edematous states

Question 200: What is the normal range for the CSF/plasma glucose ratio?

A. 1.2 - 1.6
B. 0.6 - 0.8 (Correct Answer)
C. 0.2 - 0.4
D. 1.0 - 1.2

Explanation: ***Correct: 0.6 - 0.8*** - This ratio indicates that cerebrospinal fluid (CSF) glucose concentration is typically 60-80% of plasma glucose concentration - This range is crucial for identifying metabolic or infectious pathologies affecting the central nervous system - Normal CSF glucose is approximately 50-80 mg/dL when plasma glucose is 70-120 mg/dL *Incorrect: 0.2 - 0.4* - A ratio in this range indicates significantly low CSF glucose, suggesting conditions like bacterial meningitis or hypoglycorrhachia - This is well below the normal physiological proportion of glucose in the CSF relative to plasma - Seen in bacterial/tuberculous meningitis, fungal infections, or malignancy *Incorrect: 1.0 - 1.2* - A CSF/plasma glucose ratio close to or above 1.0 would imply that CSF glucose levels are equal to or higher than plasma levels, which is physiologically impossible under normal conditions - Glucose transport into the CSF is regulated by GLUT-1 transporters and typically results in lower concentrations than in plasma - The blood-brain barrier maintains this gradient *Incorrect: 1.2 - 1.6* - This range is even more exaggerated and physiologically impossible, as CSF glucose cannot exceed plasma glucose in a healthy individual - Such a high ratio would contradict the mechanisms of glucose transport across the blood-brain barrier - Would suggest laboratory error if observed

Practice by Chapter

Body Fluid Compartments and Composition

Practice Questions

Osmolality and Tonicity

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Sodium and Water Balance

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Potassium Homeostasis

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Calcium and Phosphate Regulation

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Magnesium Metabolism

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Fluid Shifts Between Compartments

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Edema Formation Mechanisms

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Dehydration Physiology

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Disorders of Electrolyte Balance

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