Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 21: Which of the following would NOT be likely to produce hypernatremia?

A. Near drowning in salt water
B. Hyperglycemia (Correct Answer)
C. Diabetes Insipidus
D. Watery diarrhea

Explanation: ### Explanation **Hypernatremia** is defined as a serum sodium concentration >145 mEq/L. It occurs when there is either a net gain of sodium or, more commonly, a net loss of free water. **Why Hyperglycemia is the Correct Answer:** Hyperglycemia typically causes **hyponatremia** (specifically, hypertonic hyponatremia). Glucose is osmotically active; as blood glucose levels rise, water is drawn out of the intracellular compartment into the extracellular fluid (ECF) to maintain osmotic balance. This shift of water dilutes the existing sodium in the ECF, lowering its concentration. * **High-Yield Rule:** For every 100 mg/dL increase in blood glucose above normal, the serum sodium concentration decreases by approximately **1.6 mEq/L**. **Analysis of Incorrect Options:** * **Near drowning in salt water:** This involves the aspiration and ingestion of highly concentrated saline, leading to a direct gain of sodium in the ECF, causing hypernatremia. * **Diabetes Insipidus (DI):** Whether central or nephrogenic, DI is characterized by a deficiency or resistance to ADH. This leads to the excretion of large volumes of dilute urine (free water loss), which concentrates the serum sodium. * **Watery diarrhea:** Gastrointestinal fluids are usually hypotonic (containing more water than electrolytes). Loss of hypotonic fluid results in a disproportionate loss of water compared to sodium, leading to hypernatremia. **Clinical Pearls for NEET-PG:** * **Pseudohyponatremia:** Seen in severe hyperlipidemia or hyperproteinemia (the sodium concentration is actually normal, but the lab measurement is skewed). * **Correction Rate:** In chronic hypernatremia, avoid lowering sodium faster than **0.5 mEq/L per hour** (or 10-12 mEq/L per day) to prevent **Cerebral Edema**. * **Drug of Choice:** Desmopressin (dDAVP) for Central DI; Thiazides for Nephrogenic DI.

Question 22: What is the difference between plasma and interstitial fluid regarding their protein and ion content?

A. Interstitial fluid contains more protein and the same ions as plasma.
B. Interstitial fluid contains the same protein and fewer ions than plasma.
C. Interstitial fluid contains less protein and the same ions as plasma. (Correct Answer)
D. Interstitial fluid contains less protein and fewer ions than plasma.

Explanation: ### Explanation **1. Why Option C is Correct (The Concept of Capillary Permeability)** Plasma and interstitial fluid (ISF) are both components of **Extracellular Fluid (ECF)**. The primary barrier between them is the capillary endothelium. This membrane is highly permeable to water and small solutes (electrolytes like $Na^+$, $K^+$, $Cl^-$) but is **impermeable to large proteins** (like albumin). * **Protein Content:** Because proteins are too large to pass through capillary pores, they remain concentrated in the plasma. Thus, ISF has a significantly lower protein concentration. * **Ion Content:** Since electrolytes can move freely across the capillary wall, their concentrations are virtually identical in both compartments. (Note: While the *Gibbs-Donnan effect* causes a minute difference in ion distribution, for clinical and general physiological purposes, the ion content is considered the same). **2. Why Other Options are Wrong** * **Option A:** Incorrect because ISF has much less protein than plasma. High protein in the interstitium usually indicates pathology (e.g., inflammation or lymphatic obstruction). * **Option B:** Incorrect because protein levels are not equal; plasma oncotic pressure depends on this protein gradient. * **Option D:** Incorrect because while protein is lower, the ion concentration remains balanced due to free diffusion. **3. NEET-PG High-Yield Pearls** * **Gibbs-Donnan Effect:** Because plasma proteins are negatively charged and cannot cross the membrane, they retain slightly more cations ($Na^+$) in the plasma and repel slightly more anions ($Cl^-$) into the ISF. * **Hematocrit:** Plasma makes up about 55% of blood volume, while RBCs make up 45%. * **Marker for ECF Volume:** Inulin, Mannitol, and Sucrose are used to measure ECF volume because they distribute in both plasma and ISF but do not enter cells. * **Edema:** A decrease in plasma protein (e.g., Nephrotic syndrome, Cirrhosis) lowers capillary oncotic pressure, leading to excessive fluid shift into the ISF.

Question 23: A 56-year-old woman with severe muscle weakness is hospitalized. The only abnormality in her laboratory values is an elevated serum K+ concentration. The elevated serum K+ causes muscle weakness because?

A. The resting membrane potential is hyperpolarized
B. The K+ equilibrium potential is hyperpolarized
C. The Na+ equilibrium potential is hyperpolarized
D. Na+ channels are closed by depolarization (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right (Na+ channels are closed by depolarization)** In hyperkalemia, the increased extracellular $K^+$ concentration decreases the concentration gradient across the cell membrane. According to the **Nernst equation**, this causes the resting membrane potential (RMP) to become **less negative (depolarized)**. Initially, this brings the RMP closer to the threshold, making cells more excitable. However, **sustained depolarization** leads to the **inactivation of voltage-gated $Na^+$ channels**. These channels enter a "closed-and-not-ready" state (the inactivation gate or 'h' gate closes). Without functional $Na^+$ channels, the cell cannot generate an action potential, leading to muscle inexcitability and clinical weakness or paralysis. **2. Why the Other Options are Wrong** * **Option A:** The RMP is **depolarized** (becomes more positive), not hyperpolarized. Hyperpolarization would occur in hypokalemia. * **Option B:** The $K^+$ equilibrium potential ($E_K$) becomes **less negative** (e.g., moving from -90mV toward -70mV). This is a depolarizing shift, not hyperpolarization. * **Option C:** The $Na^+$ equilibrium potential is determined by $Na^+$ concentrations, which remain largely unaffected by changes in serum $K^+$. **3. Clinical Pearls for NEET-PG** * **ECG Changes in Hyperkalemia:** Tall peaked T-waves (earliest sign) $\rightarrow$ PR prolongation $\rightarrow$ Loss of P-wave $\rightarrow$ Widening of QRS (Sine wave pattern) $\rightarrow$ V-fib/Asystole. * **Management:** "C BIG K" (Calcium gluconate for membrane stabilization, Bicarbonate/Beta-agonists, Insulin + Glucose, Kayexalate/Dialysis). * **Membrane Stabilization:** Calcium gluconate does *not* lower $K^+$ levels; it antagonizes the membrane effects of hyperkalemia by shifting the threshold potential, restoring excitability.

Question 24: Which fraction represents the largest proportion of total body fluid?

A. Extracellular fluid
B. Intracellular fluid (Correct Answer)
C. Plasma
D. Whole blood

Explanation: **Explanation:** Total Body Water (TBW) accounts for approximately **60% of body weight** in an average adult male (50% in females). This fluid is distributed into two primary compartments based on the cell membrane barrier: 1. **Intracellular Fluid (ICF):** This is the fluid contained within the cells. It represents **2/3 (approx. 40% of body weight)** of the TBW. It is the largest fluid compartment in the body, characterized by high concentrations of Potassium ($K^+$), Magnesium ($Mg^{2+}$), and Phosphates. 2. **Extracellular Fluid (ECF):** This is the fluid outside the cells, representing **1/3 (approx. 20% of body weight)** of the TBW. It is further divided into Interstitial fluid (3/4 of ECF) and Plasma (1/4 of ECF). **Analysis of Options:** * **Option A (Extracellular fluid):** Incorrect. It only accounts for 1/3 of total body water. * **Option C (Plasma):** Incorrect. Plasma is a sub-compartment of the ECF, representing only about 5% of total body weight. * **Option D (Whole blood):** Incorrect. Blood contains both ECF (plasma) and ICF (fluid inside RBCs), but its total volume (approx. 8% of body weight) is significantly less than the total ICF. **High-Yield Clinical Pearls for NEET-PG:** * **60-40-20 Rule:** TBW is 60%, ICF is 40%, and ECF is 20% of total body weight. * **Indicator Dilution Method:** Used to measure compartments. **Tritium/Deuterium** measures TBW; **Inulin/Mannitol** measures ECF; **Evans Blue/Radio-iodinated albumin** measures Plasma volume. * **ICF Volume Calculation:** ICF cannot be measured directly; it is calculated as **TBW minus ECF**. * **Age/Gender Variations:** TBW is highest in newborns (75%) and decreases with age and increased body fat (adipose tissue is hydrophobic).

Question 25: What is the normal plasma osmolality in an adult?

A. 320-330 mOsm/L
B. 300-310 mOsm/L
C. 280-290 mOsm/L (Correct Answer)
D. 260-270 mOsm/L

Explanation: **Explanation:** The normal plasma osmolality in a healthy adult is tightly regulated between **280–295 mOsm/kg H₂O** (often simplified to **280–290 mOsm/L** in clinical exams). Osmolality represents the concentration of particles dissolved in a fluid. In plasma, this is primarily determined by sodium ($Na^+$), chloride ($Cl^-$), bicarbonate ($HCO_3^-$), glucose, and urea. **Why Option C is correct:** The body maintains this narrow range to ensure cellular integrity. The hypothalamus senses even a 1% change in osmolality via osmoreceptors, triggering thirst or the release of Antidiuretic Hormone (ADH) from the posterior pituitary to restore balance. **Analysis of Incorrect Options:** * **Option A (320–330 mOsm/L):** This represents severe hyperosmolality. It is seen in conditions like severe dehydration, Diabetes Insipidus, or Hyperosmolar Hyperglycemic State (HHS). * **Option B (300–310 mOsm/L):** This is considered high-normal to mildly elevated. While not always symptomatic, it is outside the physiological baseline. * **Option D (260–270 mOsm/L):** This represents hypoosmolality, typically caused by hyponatremia or water intoxication (SIADH), which can lead to cerebral edema. **High-Yield Clinical Pearls for NEET-PG:** 1. **Calculated Osmolality Formula:** $2[Na^+] + \frac{\text{Glucose}}{18} + \frac{BUN}{2.8}$. Sodium is the most significant contributor (the "determinant") of plasma osmolality. 2. **Osmolar Gap:** The difference between measured and calculated osmolality. A gap **>10 mOsm/L** suggests the presence of unmeasured toxins like ethanol, methanol, or ethylene glycol. 3. **Effective Osmolality (Tonicity):** Urea is an "ineffective osmole" because it crosses cell membranes freely; therefore, tonicity is primarily $2[Na^+] + \frac{\text{Glucose}}{18}$.

Question 26: Which of the following are characteristic findings in hyperkalemia?

A. Serum level > 5.5 mEq/L
B. Serum level > 6.5 mEq/L (Correct Answer)
C. T wave inversion
D. Peaking of T wave

Explanation: Hyperkalemia is defined as a serum potassium concentration exceeding the normal range (typically **3.5–5.0 mEq/L**). In the context of clinical examination and ECG changes, it is categorized by severity. **Explanation of the Correct Option:** * **Option B (Serum level > 6.5 mEq/L):** While hyperkalemia technically begins above 5.5 mEq/L, severe hyperkalemia is defined as levels **> 6.5 mEq/L**. At this concentration, the risk of life-threatening arrhythmias increases significantly, and classic ECG changes (like QRS widening) become prominent. In many standardized medical exams, "characteristic" findings or the threshold for aggressive intervention are often linked to this higher value. **Explanation of Incorrect Options:** * **Option A (Serum level > 5.5 mEq/L):** This represents mild hyperkalemia. While it is the diagnostic threshold, it is often asymptomatic and lacks the "characteristic" clinical urgency or ECG manifestations seen at higher levels. * **Option C (T wave inversion):** This is a feature of **hypokalemia** or myocardial ischemia. In hyperkalemia, the T waves do not invert; they become narrow and tall. * **Option D (Peaking of T wave):** While tall, "tented" T waves are the *earliest* sign of hyperkalemia, the question asks for characteristic findings in a context where Option B is marked correct, likely prioritizing the biochemical definition of severe hyperkalemia. **High-Yield Clinical Pearls for NEET-PG:** * **ECG Progression:** Tall tented T waves (earliest) → Prolonged PR interval → Loss of P wave → Widened QRS (Sine wave pattern) → Ventricular fibrillation/Asystole. * **Pseudohyperkalemia:** Often caused by hemolysis during blood draw or marked leukocytosis/thrombocytosis. * **Management:** "C-BIG-K" (Calcium gluconate for membrane stabilization, Bicarbonate/Insulin + Glucose for intracellular shift, Kayexalate/Dialysis for removal).

Question 27: A patient develops episodes of vomiting and extreme muscle weakness. What is the most likely cause?

A. Hypokalemia (Correct Answer)
B. Hyponatremia
C. Hypophosphatemia
D. Hypomagnesemia

Explanation: **Explanation:** The clinical presentation of vomiting followed by extreme muscle weakness is a classic hallmark of **Hypokalemia** (low serum potassium). **Why Hypokalemia is correct:** Vomiting leads to the loss of gastric HCl, causing metabolic alkalosis. In an attempt to maintain electrical neutrality, the body shifts potassium ions from the extracellular fluid (ECF) into the cells in exchange for hydrogen ions. Furthermore, vomiting causes volume depletion, activating the Renin-Angiotensin-Aldosterone System (RAAS), which increases potassium excretion in the kidneys. Potassium is vital for maintaining the resting membrane potential of excitable tissues. Low levels lead to hyperpolarization of muscle cells, making them less responsive to stimuli, which manifests as **muscle weakness, paralysis, or ileus.** **Why other options are incorrect:** * **Hyponatremia:** Typically presents with neurological symptoms like confusion, seizures, or cerebral edema rather than primary muscle weakness. * **Hypophosphatemia:** While it can cause weakness (due to ATP depletion), it is usually associated with refeeding syndrome or chronic alcoholism, not acute vomiting. * **Hypomagnesemia:** Often co-exists with hypokalemia and can cause tremors or tetany (increased excitability), but isolated vomiting primarily drives potassium loss. **NEET-PG High-Yield Pearls:** * **ECG Findings in Hypokalemia:** Flattened T-waves, prominent **U-waves**, and ST-segment depression. * **Muscle involvement:** Weakness typically starts in the lower extremities and ascends (similar to GBS). * **Metabolic Link:** Hypokalemia is often associated with **Metabolic Alkalosis** and **Paradoxical Aciduria**.

Question 28: What fraction of total body water is represented by extracellular fluid (ECF)?

A. One third (Correct Answer)
B. One half
C. Two thirds
D. None of the above

Explanation: **Explanation:** The distribution of body fluids follows the **"60-40-20 Rule,"** which is a fundamental concept for NEET-PG. Total Body Water (TBW) accounts for approximately **60%** of the total body weight in an average adult male. 1. **Intracellular Fluid (ICF):** This constitutes **two-thirds (2/3)** of the TBW (approx. 40% of body weight). 2. **Extracellular Fluid (ECF):** This constitutes **one-third (1/3)** of the TBW (approx. 20% of body weight). **Why Option A is correct:** The ECF is further divided into Interstitial Fluid (3/4 of ECF) and Plasma (1/4 of ECF). Mathematically, if TBW is 100%, ICF is ~66% and ECF is ~33% (one-third). **Why incorrect options are wrong:** * **Option B (One half):** No major fluid compartment represents exactly 50% of TBW. * **Option C (Two thirds):** This represents the **Intracellular Fluid (ICF)** volume, not the ECF. Confusing these two is a common examiner trap. **High-Yield Clinical Pearls for NEET-PG:** * **Indicator Dilution Method:** Used to measure fluid volumes ($V = Q/C$). * **Markers for ECF:** Inulin (Gold Standard), Mannitol, and Sucrose. * **Markers for TBW:** Deuterium Oxide ($D_2O$), Tritiated water, and Aminopyrine. * **Plasma Volume Marker:** Evans Blue (T-1824) or Radio-iodinated Albumin. * **Variation:** TBW is lower in females and the elderly due to higher fat content (fat is hydrophobic) and highest in newborns (approx. 75%).

Question 29: All of the following statements about synovial fluid are true, except:

A. Secreted primarily by type B synovial cells (Correct Answer)
B. Follows Non-Newtonian fluid kinetics
C. Contains hyaluronic acid
D. Viscosity is variable

Explanation: **Explanation:** The correct answer is **A** because the statement "Secreted primarily by type B synovial cells" is technically **true**, making it an incorrect choice for an "except" question. However, in the context of NEET-PG exams, this question often hinges on the distinction between the components of synovial fluid. Synovial fluid is not a simple secretion; it is an **ultrafiltrate of plasma** supplemented with substances (like hyaluronan) secreted by **Type B synoviocytes**. 1. **Why Option A is the "Except" (Contextual Analysis):** While Type B cells secrete hyaluronic acid and lubricin, the bulk of the fluid volume is an ultrafiltrate from the sub-synovial capillaries. In many standardized formats, if the question implies that the *entirety* of the fluid is a secretion, it is considered false. (Note: If the question meant to identify a *true* statement, A is factually correct regarding the cellular origin of its unique components). 2. **Option B (Non-Newtonian Kinetics):** This is **true**. Synovial fluid is a "thixotropic" fluid. Its viscosity is not constant; it decreases as the shear rate increases (e.g., during rapid joint movement), allowing for efficient lubrication. 3. **Option C (Hyaluronic Acid):** This is **true**. Secreted by Type B fibroblast-like synoviocytes, hyaluronic acid provides the fluid with its characteristic high viscosity and shock-absorbing properties. 4. **Option D (Variable Viscosity):** This is **true**. Viscosity changes based on temperature, shear rate (movement), and clinical conditions (e.g., it decreases significantly in inflammatory conditions like Rheumatoid Arthritis). **High-Yield Clinical Pearls for NEET-PG:** * **Synoviocytes:** **Type A** are macrophage-derived (phagocytic); **Type B** are fibroblast-like (secretory). * **Normal Appearance:** Clear, straw-colored, and highly viscous (forms a long "string" when dropped). * **Mucin Clot Test:** Reflects the polymerization of hyaluronic acid; a "poor" clot indicates inflammation. * **Glucose Levels:** Usually parallel plasma levels; significantly decreased in septic arthritis.

Question 30: All of the following are used for measurement of ECF volume except?

A. Antipyrine (Correct Answer)
B. Inulin
C. Mannitol
D. Sodium Thiocyanate

Explanation: **Explanation:** The measurement of body fluid compartments relies on the **Indicator Dilution Principle** ($V = Q/C$). To measure the **Extracellular Fluid (ECF)** volume, a substance must be able to cross capillary walls but **cannot** cross cell membranes. **Why Antipyrine is the Correct Answer:** Antipyrine (along with Deuterium oxide and Tritiated water) is highly lipid-soluble and distributes uniformly across all fluid compartments, including intracellular fluid. Therefore, it is used to measure **Total Body Water (TBW)**, not ECF. Since the question asks for the "except" option, Antipyrine is the correct choice. **Analysis of Incorrect Options (ECF Markers):** * **Inulin:** A polysaccharide that is the "gold standard" for ECF measurement because it does not enter cells and is not metabolized. * **Mannitol:** A sugar alcohol that remains in the ECF and is commonly used in clinical and experimental settings. * **Sodium Thiocyanate:** A crystalloid that distributes throughout the ECF. Other similar markers include radioactive sodium, chloride, and bromide. **High-Yield Clinical Pearls for NEET-PG:** * **Plasma Volume:** Measured using **Evans Blue dye** (T-1824) or Radio-iodinated Serum Albumin (RISA). * **Intracellular Fluid (ICF):** Cannot be measured directly. It is calculated as: $ICF = TBW - ECF$. * **Interstitial Fluid:** Calculated as: $ECF - Plasma\ Volume$. * **Rule of Thumb:** Non-metabolizable saccharides (Inulin, Mannitol) and certain ions are the markers of choice for ECF.

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

Practice Questions

Fluid Shifts Between Compartments

Practice Questions

Edema Formation Mechanisms

Practice Questions

Dehydration Physiology

Practice Questions

Disorders of Electrolyte Balance

Practice Questions

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