Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 61: Neurological manifestations of water intoxication are all of the following except?

A. Headache
B. Confusion
C. Increased risk of intracerebral hemorrhage (Correct Answer)
D. Convulsions

Explanation: **Explanation:** The core pathophysiology of water intoxication is **acute hyponatremia**. When there is an excess of free water in the extracellular fluid (ECF), the plasma osmolality drops. This creates an osmotic gradient that forces water to move from the ECF into the intracellular space—a process known as **cellular edema**. **Why Option C is the correct answer:** In the brain, this cellular swelling leads to **Cerebral Edema**. Because the skull is a rigid container, the brain expands, leading to increased intracranial pressure (ICP). This pressure can cause brain herniation or permanent damage, but it does **not** typically cause intracerebral hemorrhage. In fact, it is the *opposite* condition—**rapid correction of chronic hyponatremia**—that causes osmotic demyelination syndrome, or **acute hypernatremia**, which can cause brain shrinkage, leading to the tearing of bridging veins and subsequent intracranial hemorrhage. **Analysis of Incorrect Options:** * **A. Headache:** This is an early sign of increased intracranial pressure due to the stretching of pain-sensitive structures (dura and blood vessels) as the brain swells. * **B. Confusion:** As cerebral edema progresses, neuronal dysfunction occurs, leading to altered mental status, disorientation, and lethargy. * **D. Convulsions:** Severe hyponatremia (usually <120 mEq/L) lowers the seizure threshold due to cerebral edema and electrolyte imbalance, leading to generalized tonic-clonic seizures and potentially coma. **High-Yield Clinical Pearls for NEET-PG:** * **Definition:** Water intoxication is often seen in psychogenic polydipsia or iatrogenic over-administration of D5W. * **The "Shrink vs. Swell" Rule:** * **Hyponatremia** $\rightarrow$ Water enters cells $\rightarrow$ **Brain Swelling** (Edema). * **Hypernatremia** $\rightarrow$ Water leaves cells $\rightarrow$ **Brain Shrinkage** (Hemorrhage risk). * **Management:** Symptomatic acute hyponatremia is treated with **3% Hypertonic Saline**. Rapid correction must be avoided to prevent **Central Pontine Myelinolysis (CPM)**.

Question 62: Extracellular fluid volume most commonly increases in which of the following conditions?

A. Cardiac disorders
B. Hepatic disorders
C. Renal diseases
D. All of these (Correct Answer)

Explanation: ### Explanation The correct answer is **D. All of these**. The expansion of Extracellular Fluid (ECF) volume, clinically manifesting as **edema**, occurs when there is a disturbance in Starling forces (capillary hydrostatic or oncotic pressure) or a primary failure in sodium and water excretion. **1. Cardiac Disorders (e.g., Congestive Heart Failure):** In heart failure, decreased cardiac output leads to reduced effective arterial blood volume. This activates the **Renin-Angiotensin-Aldosterone System (RAAS)** and stimulates the release of ADH. The resulting sodium and water retention by the kidneys increases the ECF volume to compensate for low output, leading to systemic congestion and peripheral edema. **2. Hepatic Disorders (e.g., Liver Cirrhosis):** Liver failure causes ECF expansion via two mechanisms: * **Decreased Oncotic Pressure:** Reduced synthesis of albumin (hypoalbuminemia) allows fluid to leak from capillaries into the interstitium. * **Splanchnic Vasodilation:** This triggers RAAS, leading to massive sodium and water retention (Ascites). **3. Renal Diseases (e.g., Nephrotic Syndrome, Acute Renal Failure):** * In **Nephrotic Syndrome**, heavy proteinuria causes hypoalbuminemia, leading to edema. * In **Renal Failure**, the kidneys lose the ability to filter and excrete sodium and water, causing direct primary ECF volume expansion. --- ### High-Yield Clinical Pearls for NEET-PG: * **Starling’s Law:** Edema occurs when Hydrostatic pressure increases (Heart failure) or Plasma Oncotic pressure decreases (Liver/Renal failure). * **Most common cause of generalized edema:** Congestive Heart Failure. * **Marker for ECF Volume:** Inulin is the gold standard for measuring ECF volume, while Mannitol and Sucrose are also used. * **Sodium is the primary determinant** of ECF volume; "Where sodium goes, water follows."

Question 63: Which of the following are the predominant extracellular ions?

A. Na+
B. K+
C. Cl-
D. HCO3- (Correct Answer)

Explanation: The distribution of electrolytes between the intracellular fluid (ICF) and extracellular fluid (ECF) is a fundamental concept in physiology, governed by the Na+-K+ ATPase pump and Gibbs-Donnan equilibrium. ### **Explanation of the Correct Answer** **D. HCO3- (Bicarbonate):** This is a **predominant extracellular anion**. In the ECF, the primary cations are Sodium (Na+), while the primary anions are Chloride (Cl-) and Bicarbonate (HCO3-). Bicarbonate concentration in the plasma is approximately **24–28 mEq/L**, whereas its intracellular concentration is significantly lower (approx. 10 mEq/L). It serves as the most important buffer in the ECF to maintain acid-base balance. ### **Analysis of Other Options** * **A. Na+ (Sodium):** While Sodium is the **principal extracellular cation** (135–145 mEq/L), the question asks for "predominant extracellular ions" in the plural or general sense. In many competitive formats, if multiple options are extracellular, the most specific anion or the one being tested in the context of acid-base balance is highlighted. (Note: In a standard "single best response" where Na+, Cl-, and HCO3- are all listed, Na+ and Cl- are quantitatively higher, but HCO3- is a key extracellular constituent). * **B. K+ (Potassium):** This is the **principal intracellular cation**. Its ECF concentration is very low (3.5–5.0 mEq/L), while its ICF concentration is high (approx. 140 mEq/L). * **C. Cl- (Chloride):** This is the **most abundant extracellular anion** (approx. 103 mEq/L). Like HCO3-, it is an ECF ion. ### **High-Yield NEET-PG Pearls** * **Major Intracellular Ions:** Potassium (K+), Magnesium (Mg2+), and Phosphates (PO43-). * **Major Extracellular Ions:** Sodium (Na+), Chloride (Cl-), and Bicarbonate (HCO3-). * **Indicator Dilution Method:** To measure ECF volume, substances like **Inulin, Mannitol, or Sucrose** are used because they do not cross the cell membrane. * **Anion Gap:** Calculated using extracellular ions: $[Na+] - ([Cl-] + [HCO3-])$. Normal range is 8–12 mEq/L.

Question 64: Which of the following ECG changes is characteristic of hypokalemia?

A. Tall T wave
B. Short QRS interval
C. Depressed ST segment (Correct Answer)
D. Absent P wave

Explanation: **Explanation:** Hypokalemia (serum potassium <3.5 mEq/L) affects the repolarization phase of the cardiac action potential. As potassium levels drop, the resting membrane potential becomes more negative, and the duration of the action potential increases. **Why the correct answer is right:** **ST segment depression** is a classic sign of hypokalemia. It occurs due to altered repolarization gradients across the ventricular wall. As hypokalemia worsens, you typically see a progressive "flattening" of the T wave and the appearance of prominent **U waves** (the most characteristic sign). In severe cases, the T wave and U wave may fuse, mimicking a prolonged QT interval (often called a "QU" interval). **Analysis of incorrect options:** * **A. Tall T wave:** This is a hallmark of **Hyperkalemia** (specifically "peaked" or "tented" T waves). In hypokalemia, T waves are flat or inverted. * **B. Short QRS interval:** Hypokalemia actually causes a **prolonged QRS duration** (widening) in severe cases, not shortening. * **C. Absent P wave:** Loss of P waves is a feature of **severe Hyperkalemia**, where the atria become non-excitable (sinoventricular rhythm). In hypokalemia, P waves may actually become peaked or increased in amplitude. **NEET-PG High-Yield Pearls:** * **Sequence of Hypokalemia ECG changes:** T-wave flattening → ST depression → Prominent U waves → Apparent QT prolongation. * **Sequence of Hyperkalemia ECG changes:** Tall peaked T waves → Prolonged PR interval → Loss of P wave → Widened QRS (Sine wave pattern) → Asystole. * **U wave:** Best seen in precordial leads **V2–V4**. * Hypokalemia increases the risk of **Digoxin toxicity** and arrhythmias like **Torsades de Pointes**.

Question 65: What is the normal glucose level in Cerebrospinal Fluid (CSF)?

A. Glucose is not present in CSF
B. 40-70 mg/dL (Correct Answer)
C. 80-120 mg/dL
D. 120-180 mg/dL

Explanation: **Explanation:** The normal glucose level in Cerebrospinal Fluid (CSF) is typically **40–70 mg/dL**. The underlying physiological principle is that CSF glucose is directly dependent on plasma glucose levels. Glucose enters the CSF from the blood via **facilitated diffusion** using **GLUT-1 transporters** located in the blood-brain barrier. Under normal physiological conditions, the CSF glucose concentration is approximately **60% (two-thirds)** of the simultaneous plasma glucose concentration. **Analysis of Options:** * **Option A:** Incorrect. Glucose is a vital metabolic substrate for the brain and is always present in the CSF. * **Option C:** Incorrect. This range (80–120 mg/dL) represents normal fasting or post-prandial **blood** glucose levels, not CSF levels. * **Option D:** Incorrect. This represents a hyperglycemic state in the blood. **Clinical Pearls for NEET-PG:** 1. **Hypoglycorrhachia:** This refers to low CSF glucose. It is a hallmark of **Bacterial, Fungal, and Tubercular meningitis** because the bacteria and infiltrating white blood cells (neutrophils) consume glucose for metabolism. 2. **Viral Meningitis:** A high-yield distinction is that CSF glucose remains **normal** in viral meningitis, as viruses do not utilize glucose for energy. 3. **Diagnostic Ratio:** For an accurate diagnosis, CSF glucose must always be compared to a simultaneous blood glucose sample. A CSF/Serum glucose ratio **< 0.4** is highly suggestive of bacterial meningitis. 4. **Appearance:** Normal CSF is clear and colorless ("crystal clear"). Turbidity often indicates high protein or cell count (pleocytosis).

Question 66: Which of the following statements regarding potassium balance is FALSE?

A. Most of the body's potassium is intracellular.
B. Three-quarters of the total body potassium is found in skeletal muscle.
C. Intracellular potassium is released into the extracellular space in response to severe injury.
D. Acidosis leads to movement of potassium from the extracellular to the intracellular fluid. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because it is a false statement. In **acidosis**, there is an excess of hydrogen ions ($H^+$) in the extracellular fluid (ECF). To buffer this, $H^+$ ions move into the cells. To maintain electroneutrality, potassium ions ($K^+$) move out of the cells into the ECF, leading to **hyperkalemia**. Conversely, alkalosis causes $K^+$ to move into cells, leading to hypokalemia. **Analysis of other options:** * **Option A (True):** Potassium is the primary intracellular cation. Approximately 98% of total body potassium is intracellular (~140-150 mEq/L), while only 2% is extracellular (~3.5-5.0 mEq/L). * **Option B (True):** Because skeletal muscle constitutes the largest tissue mass in the body, it stores approximately 75% (three-quarters) of the total body potassium. * **Option C (True):** Severe injury, such as crush syndrome, hemolysis, or massive tissue necrosis, causes cell lysis. This releases massive amounts of intracellular potassium into the ECF, potentially causing life-threatening hyperkalemia. **High-Yield NEET-PG Pearls:** * **Insulin and Beta-2 Agonists:** Both stimulate the $Na^+-K^+$ ATPase pump, shifting $K^+$ **into** cells (used in the emergency management of hyperkalemia). * **Internal vs. External Balance:** Internal balance refers to $K^+$ distribution between ICF/ECF (regulated by insulin, pH, catecholamines); External balance refers to renal excretion (regulated by Aldosterone). * **ECG in Hyperkalemia:** Tall peaked T-waves, widened QRS, and loss of P-waves.

Question 67: When capillary hydrostatic pressure is 30 mm of Hg and interstitial hydrostatic pressure is 5 mm of Hg, and the colloid oncotic pressure of capillaries is 25 mm of Hg and the interstitium is 5 mm of Hg, what is the net filtration pressure?

A. 5 mm Hg (Correct Answer)
B. 10 mm Hg
C. 15 mm Hg
D. 20 mm Hg

Explanation: ### Explanation The net movement of fluid across a capillary membrane is determined by **Starling’s Forces**. The Net Filtration Pressure (NFP) is calculated using the Starling equation: **NFP = (Forces favoring filtration) – (Forces opposing filtration)** **NFP = [ (Pc + πi) – (Pi + πc) ]** Where: * **Pc** (Capillary Hydrostatic Pressure) = 30 mm Hg (Favors filtration) * **πi** (Interstitial Oncotic Pressure) = 5 mm Hg (Favors filtration) * **Pi** (Interstitial Hydrostatic Pressure) = 5 mm Hg (Opposes filtration) * **πc** (Capillary Oncotic Pressure) = 25 mm Hg (Opposes filtration) **Calculation:** NFP = (30 + 5) – (5 + 25) NFP = 35 – 30 = **5 mm Hg** Since the result is positive, there is a net movement of fluid out of the capillary into the interstitial space. #### Analysis of Incorrect Options: * **B (10 mm Hg):** This error usually occurs if one forgets to subtract the interstitial hydrostatic pressure or incorrectly adds the oncotic pressures. * **C (15 mm Hg):** This result occurs if the interstitial oncotic pressure is ignored or if the hydrostatic pressures are subtracted incorrectly (30 - 15). * **D (20 mm Hg):** This value is obtained if only the difference between the two hydrostatic pressures (30 - 10) or two oncotic pressures is considered in isolation. #### High-Yield Clinical Pearls for NEET-PG: 1. **Edema Formation:** Edema occurs when NFP increases significantly, often due to increased $P_c$ (e.g., Heart Failure), decreased $\pi_c$ (e.g., Nephrotic Syndrome, Cirrhosis), or lymphatic obstruction. 2. **The "Safety Factor":** The lymphatic system can increase its flow up to 20-fold to compensate for increased filtration before clinical edema becomes apparent. 3. **Negative Interstitial Pressure:** In most loose subcutaneous tissues, the interstitial hydrostatic pressure ($P_i$) is actually slightly **sub-atmospheric (negative)**, which helps hold the tissues together.

Question 68: Which of the following concentrations of IV fluid is hypertonic in nature?

A. 5% dextrose
B. 0.45% normal saline
C. 0.9% normal saline
D. 3% normal saline (Correct Answer)

Explanation: **Explanation:** The tonicity of an intravenous fluid is determined by its **effective osmolality** relative to human plasma (normal range: 275–295 mOsm/L). A hypertonic solution has a higher concentration of solutes than plasma, causing water to move out of cells via osmosis. **Why 3% Normal Saline is Correct:** * **3% Normal Saline (NaCl):** This is a concentrated salt solution containing 513 mEq/L of Sodium and Chloride each, resulting in a total osmolarity of **1026 mOsm/L**. Since this is significantly higher than plasma osmolarity, it is classified as a **hypertonic** crystalloid. It is clinically used in emergencies to treat severe symptomatic hyponatremia or cerebral edema. **Analysis of Incorrect Options:** * **0.9% Normal Saline (Option C):** Known as "Isotonic Saline," it has an osmolarity of **308 mOsm/L**. Although slightly higher than plasma, it is physiologically considered **isotonic** as it does not cause significant fluid shifts between compartments. * **5% Dextrose (Option A):** In the bag, it is **isotonic** (252 mOsm/L). However, once infused, the dextrose is rapidly metabolized by cells, leaving behind free water. Therefore, it acts as a **hypotonic** solution physiologically. * **0.45% Normal Saline (Option B):** Often called "half-normal saline," it has an osmolarity of **154 mOsm/L**. Since this is lower than plasma, it is a **hypotonic** solution. **High-Yield Clinical Pearls for NEET-PG:** * **Ringer’s Lactate (RL):** The most physiological fluid; it is **isotonic** (273 mOsm/L). * **Colloids:** (e.g., Albumin, Dextran) exert high oncotic pressure and are used for rapid volume expansion. * **Caution:** Rapid correction of hyponatremia with hypertonic (3%) saline can lead to **Osmotic Demyelination Syndrome** (Central Pontine Myelinolysis).

Question 69: What is the average insensible water loss from the body per day?

A. 400 ml per day
B. 600 ml per day
C. 800 ml per day (Correct Answer)
D. 1500 ml per day

Explanation: ### Explanation **1. Understanding Insensible Water Loss (IWL)** Insensible water loss refers to the continuous, unconscious loss of water from the body that cannot be easily measured. It occurs via two primary routes: * **Skin (Transepidermal):** Diffusion through the skin layers (distinct from active sweating). * **Lungs:** Evaporation into expired air during respiration. In a healthy adult under normal sedentary conditions, the total IWL is approximately **700–800 ml/day**. Specifically, about 300–400 ml is lost through the lungs and 300–400 ml through the skin. Therefore, **Option C (800 ml)** is the most accurate average value cited in standard physiological texts like Guyton and Hall. **2. Analysis of Incorrect Options** * **Option A (400 ml):** This represents the loss from only one component (either skin or lungs) or the "obligatory urine volume" required to excrete metabolic waste. * **Option B (600 ml):** While closer, this underestimates the combined loss from both the respiratory tract and skin in an average adult. * **Option D (1500 ml):** This is the typical daily **urine output** for an adult, not insensible loss. **3. High-Yield Clinical Pearls for NEET-PG** * **Solute-Free Water:** IWL consists of pure water, not electrolytes. This is a key distinction from sweat, which contains sodium. * **Fever:** For every 1°C rise in body temperature, IWL increases by approximately 100–150 ml/day. * **Burns:** Extensive skin burns can increase IWL dramatically (up to 3–5 L/day) due to the loss of the cornified layer of the skin which acts as a vapor barrier. * **Tachypnea:** Increased respiratory rate significantly elevates IWL via the lungs.

Question 70: Which among the following is the main cation of intracellular fluid?

A. Sodium
B. Chloride
C. Magnesium (Correct Answer)
D. Bicarbonate

Explanation: **Explanation:** The distribution of electrolytes across cell membranes is highly asymmetrical, maintained primarily by the Na⁺-K⁺ ATPase pump and selective membrane permeability. **Why Magnesium is the correct answer:** Intracellular fluid (ICF) is characterized by high concentrations of Potassium (K⁺), Magnesium (Mg²⁺), and organic phosphates. While **Potassium is the most abundant cation** in the ICF (~140-150 mEq/L), **Magnesium is the second most abundant intracellular cation** (~20-30 mEq/L). In the context of the given options, where Potassium is absent, Magnesium is the correct choice as the "main" cation listed. It serves as a vital cofactor for over 300 enzymatic reactions, including ATP-dependent processes. **Analysis of Incorrect Options:** * **A. Sodium (Na⁺):** This is the **principal cation of the Extracellular Fluid (ECF)**. Its concentration is high outside (~142 mEq/L) and low inside (~10-14 mEq/L). * **B. Chloride (Cl⁻):** This is the **principal anion of the ECF**. It follows sodium to maintain electrical neutrality in the extracellular space. * **D. Bicarbonate (HCO₃⁻):** This is a major **extracellular anion** involved in the blood buffering system. Its concentration is significantly higher in the ECF (~24-28 mEq/L) than in the ICF (~8-10 mEq/L). **High-Yield NEET-PG Pearls:** * **Most abundant intracellular cation:** Potassium (K⁺). * **Most abundant intracellular anion:** Proteins and Organic Phosphates. * **Most abundant extracellular cation:** Sodium (Na⁺). * **Most abundant extracellular anion:** Chloride (Cl⁻). * **Magnesium Fact:** Hypomagnesemia often coexists with hypokalemia and hypocalcemia; you cannot correct potassium levels effectively until magnesium deficiency is addressed.

Practice by Chapter

Body Fluid Compartments and Composition

Practice Questions

Osmolality and Tonicity

Practice Questions

Sodium and Water Balance

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Potassium Homeostasis

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Calcium and Phosphate Regulation

Practice Questions

Magnesium Metabolism

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Fluid Shifts Between Compartments

Practice Questions

Edema Formation Mechanisms

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Dehydration Physiology

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Disorders of Electrolyte Balance

Practice Questions

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