Electrolytes and Body Fluids Practice Questions

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

All of these. ### 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."

Q: Which of the following would be the plasma osmolality of a child with plasma Na+ 125 mEq/L, glucose of 108 mg/dL, and blood urea nitrogen (BUN) of 140 mg/dL?

306 mOsm/kg. To calculate the plasma osmolality, we use the standard clinical formula which accounts for the primary osmotically active particles in the blood: Sodium, Glucose, and Blood Urea Nitrogen (BUN). ### **The Formula** **Calculated Plasma Osmolality = 2 × [Na⁺] + [Glucose / 18] + [BUN / 2.8]** ### **Step-by-Step Calculation** 1. **Sodium component:** 2 × 125 = **250** 2. **Glucose component:** 108 / 18 = **6** 3. **BUN component:** 140 / 2.8 = **50** 4. **Total:** 250 + 6 + 50 = **306 mOsm/kg** ### **Why Option B is Correct** Option B (**306 mOsm/kg**) is the precise result of the calculation. Despite the child having hyponatremia (Na+ 125), the significantly elevated BUN (140 mg/dL)—likely due to renal failure—contributes substantially to the total osmolality, bringing it into the hyperosmolar range. ### **Why Other Options are Incorrect** * **Options A, C, and D** are incorrect because they result from mathematical errors or using incorrect divisors (e.g., forgetting to divide glucose by 18 or BUN by 2.8). In NEET-PG, examiners often provide options that would be reached if a student used 5.6 instead of 2.8 for BUN or failed to double the Sodium value. ### **High-Yield Clinical Pearls for NEET-PG** * **Effective Osmolality (Tonicity):** This excludes urea because urea is a "permeable solute" that crosses cell membranes freely. The formula for effective osmolality is: **2 × [Na⁺] + [Glucose / 18]**. * **Osmolar Gap:** This is the difference between *measured* osmolality (via laboratory osmometer) and *calculated* osmolality. A gap >10 mOsm/L suggests the presence of unmeasured substances like ethanol, methanol, or ethylene glycol. * **Normal Range:** Normal plasma osmolality typically ranges from **275–295 mOsm/kg**.

Q: Which of the following is NOT caused by hypokalemia?

Tetany. **Explanation:** **1. Why Tetany is the Correct Answer:** Tetany is a state of increased neuromuscular excitability characterized by involuntary muscle spasms. It is classically caused by **hypocalcemia**, **hypomagnesemia**, or **alkalosis**. Hypokalemia, conversely, leads to **hyperpolarization** of the resting membrane potential (making it more negative). This moves the cell further away from the threshold potential, making it *less* excitable. Therefore, hypokalemia typically causes muscle weakness and paralysis, not tetany. **2. Analysis of Incorrect Options:** * **Paralytic Ileus:** Low potassium levels decrease the excitability and motility of smooth muscles in the gastrointestinal tract, leading to intestinal atony and paralytic ileus. * **Hypertension:** Chronic hypokalemia (often associated with high salt intake or primary hyperaldosteronism) can lead to vasoconstriction and increased renal sodium reabsorption, contributing to elevated blood pressure. * **Rhabdomyolysis:** Potassium is a profound vasodilator in skeletal muscle during exercise. In severe hypokalemia, the failure of muscular vasodilation leads to ischemia and subsequent muscle cell breakdown (rhabdomyolysis). **3. Clinical Pearls for NEET-PG:** * **ECG Findings in Hypokalemia:** Flattened T-waves, **prominent U-waves**, ST-segment depression, and prolonged PR interval (High-yield: "U wave is after the T wave"). * **Muscle Effects:** Hypokalemia causes "floppy" muscles (flaccid paralysis), whereas hyperkalemia can cause "peaked T waves" and cardiac arrest in diastole. * **Refractory Hypokalemia:** If hypokalemia does not respond to potassium supplementation, always check **Magnesium** levels; hypomagnesemia must be corrected first.

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

3% normal saline. **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).

Q: Which of the following has the greatest effect on plasma osmolality?

Vasopressin. **Explanation:** **Correct Answer: C. Vasopressin (Antidiuretic Hormone - ADH)** Vasopressin is the primary regulator of **plasma osmolality**. It acts on the V2 receptors in the late distal tubule and collecting ducts of the kidney to insert aquaporin-2 channels. This allows for the selective reabsorption of **free water** without solutes. Because osmolality is defined as the concentration of solutes per kilogram of solvent, the addition or removal of free water is the most efficient way to dilute or concentrate the plasma, thereby directly controlling osmolality. **Why the other options are incorrect:** * **Aldosterone (D):** While aldosterone increases sodium reabsorption, it does so via an "isosmotic" process (water follows sodium). Therefore, it primarily affects **Extracellular Fluid (ECF) volume** rather than osmolality. * **Cortisol (B):** This is a glucocorticoid primarily involved in glucose metabolism and the stress response. While it has minor mineralocorticoid activity at high levels, its effect on osmolality is negligible compared to ADH. * **Progesterone (A):** This is a reproductive hormone. While it can compete with aldosterone at the mineralocorticoid receptor (leading to mild natriuresis), it plays no significant role in the acute regulation of plasma osmolality. **High-Yield Clinical Pearls for NEET-PG:** * **Osmoreceptors:** Located in the anterior hypothalamus (OVLT and SFO), they sense a change as small as **1%** in osmolality to trigger ADH release. * **Volume vs. Osmolality:** ADH regulates **Osmolality** (via water); Aldosterone regulates **Volume** (via sodium). * **SIADH:** Characterized by excessive ADH, leading to hyponatremia and low plasma osmolality. * **Diabetes Insipidus:** Characterized by ADH deficiency or resistance, leading to hypernatremia and high plasma osmolality.

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

800 ml per day. ### 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.

Q: Dehydration increases the plasma concentration of all the following hormones EXCEPT?

Atrial natriuretic peptide. **Explanation:** The correct answer is **Atrial Natriuretic Peptide (ANP)**. Dehydration leads to a decrease in total blood volume (hypovolemia) and an increase in plasma osmolarity. The body’s primary goal during dehydration is to conserve water and sodium to restore blood pressure and volume. **Why ANP is the correct answer:** ANP is a hormone secreted by the atrial myocytes in response to **atrial stretch** (increased venous return/volume overload). Its primary function is to promote natriuresis (sodium excretion) and diuresis (water excretion) to lower blood pressure. In dehydration, the atria are "under-filled," leading to **decreased** atrial stretch; therefore, ANP secretion is inhibited. **Why the other options are incorrect:** * **Vasopressin (ADH):** Dehydration increases plasma osmolarity, which is sensed by hypothalamic osmoreceptors. This triggers the release of ADH to increase water reabsorption in the collecting ducts. * **Angiotensin II:** Hypovolemia causes decreased renal perfusion, stimulating the Juxtaglomerular (JG) apparatus to release Renin. Renin converts Angiotensinogen to Angiotensin I, which is then converted to Angiotensin II. * **Aldosterone:** Angiotensin II and increased plasma potassium stimulate the adrenal cortex to release Aldosterone, which promotes sodium and water retention to restore volume. **High-Yield Clinical Pearls for NEET-PG:** * **ANP vs. ADH:** Think of them as antagonists. ANP "wastes" salt/water (lowers BP), while ADH/Aldosterone "save" salt/water (raises BP). * **BNP (B-type Natriuretic Peptide):** Secreted by ventricles in response to pressure/volume overload; used clinically as a marker for Heart Failure. * **Stimulus for ADH:** Increased osmolarity is a more sensitive stimulus for ADH release than decreased volume.

Q: Extracellular fluid (ECF) volume can be measured using which of the following substances?

Inulin. **Explanation:** The measurement of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = Amount / Concentration$). To measure a specific compartment, the substance used must be able to distribute evenly within that compartment without entering others or being metabolized rapidly. **Why Inulin is Correct:** **Inulin** is a polysaccharide that is small enough to pass through capillary endothelium into the interstitial space but is too large to cross cell membranes. Therefore, it distributes throughout the entire **Extracellular Fluid (ECF)**—which includes both plasma and interstitial fluid—making it the gold standard for ECF volume measurement. Other substances used for ECF include Mannitol, Sucrose, and Thiosulfate. **Why Other Options are Incorrect:** * **Deuterium oxide ($D_2O$) and Tritiated water ($^3H_2O$):** These are isotopes of water. They cross all cell membranes and distribute uniformly throughout the **Total Body Water (TBW)**. * **Evan’s Blue (T-1824):** This dye binds strongly to serum albumin. Since albumin is largely confined to the vascular system, Evan’s Blue is used to measure **Plasma Volume**. **High-Yield Clinical Pearls for NEET-PG:** * **Total Body Water (60% of body weight):** Measured by $D_2O$, Tritiated water, or Aminopyrine. * **ECF (20% of body weight):** Measured by Inulin, Mannitol, or $Na^{22}$. * **Plasma Volume (5% of body weight):** Measured by Evan’s Blue or Radio-iodinated Serum Albumin (RISA). * **Calculation Rule:** Intracellular Fluid (ICF) and Interstitial Fluid (ISF) cannot be measured directly. They are calculated: * $ICF = TBW - ECF$ * $ISF = ECF - Plasma\ Volume$

Q: The percentage of body water is greater in which group?

Males than in females. **Explanation:** Total Body Water (TBW) is inversely proportional to body fat content. Adipose tissue is hydrophobic and contains very little water (approx. 10%), whereas lean muscle mass is hydrophilic and contains significant water (approx. 75%). Therefore, individuals with higher muscle mass and lower fat percentages have a higher percentage of TBW. **Why Option A is Correct:** Adult males typically have a higher proportion of lean muscle mass and lower subcutaneous fat compared to adult females (due to the effects of testosterone vs. estrogen). Consequently, TBW is approximately **60%** of body weight in males and **50%** in females. **Analysis of Incorrect Options:** * **B. Children than in adults:** This is actually a factually correct statement (Infants have ~75% TBW). However, in the context of standard NEET-PG MCQ patterns, when comparing gender vs. age, the physiological baseline for "greater" usually refers to the male/female dichotomy unless "Infants" is specifically specified. *Note: If this were a "Multiple Correct" scenario, B would also be true.* * **C. Obese than in lean:** Incorrect. Obese individuals have more adipose tissue (fat), which displaces water, leading to a lower percentage of TBW compared to lean individuals. * **D. Old than in young:** Incorrect. As age increases, muscle mass decreases (sarcopenia) and fat percentage typically increases, leading to a progressive decline in TBW. **Clinical Pearls for NEET-PG:** * **Highest TBW:** Premature infants (~80%) > Term neonates (~70-75%). * **Standard TBW:** 60% (0.6 × body weight). * **Fluid Compartments:** TBW is divided into Intracellular Fluid (ICF = 2/3 or 40% of body weight) and Extracellular Fluid (ECF = 1/3 or 20% of body weight). * **Rule of Thumb:** Fat is "dry," muscle is "wet." Any condition increasing fat decreases the percentage of body water.

Question 1

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

Accepted Answer

All of these

Answer

Cardiac disorders

Answer

Hepatic disorders

Answer

Renal diseases

Question 2

Which of the following would be the plasma osmolality of a child with plasma Na+ 125 mEq/L, glucose of 108 mg/dL, and blood urea nitrogen (BUN) of 140 mg/dL?

Accepted Answer

306 mOsm/kg

Answer

360 mOsm/kg

Answer

312 mOsm/kg

Answer

318 mOsm/kg

Question 3

Which of the following statements regarding potassium balance is FALSE?

Accepted Answer

Acidosis leads to movement of potassium from the extracellular to the intracellular fluid.

Answer

Most of the body's potassium is intracellular.

Answer

Three-quarters of the total body potassium is found in skeletal muscle.

Answer

Intracellular potassium is released into the extracellular space in response to severe injury.

Question 4

Which of the following is NOT caused by hypokalemia?

Accepted Answer

Tetany

Answer

Paralytic ileus

Answer

Hypertension

Answer

Rhabdomyolysis

Question 5

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

Accepted Answer

3% normal saline

Answer

5% dextrose

Answer

0.45% normal saline

Answer

0.9% normal saline

Question 6

Which of the following has the greatest effect on plasma osmolality?

Accepted Answer

Vasopressin

Answer

Progesterone

Answer

Cortisol

Answer

Aldosterone

Question 7

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

Accepted Answer

800 ml per day

Answer

400 ml per day

Answer

600 ml per day

Answer

1500 ml per day

Question 8

Dehydration increases the plasma concentration of all the following hormones EXCEPT?

Accepted Answer

Atrial natriuretic peptide

Answer

Vasopressin

Answer

Aldosterone

Answer

Angiotensin II

Question 9

Extracellular fluid (ECF) volume can be measured using which of the following substances?

Accepted Answer

Inulin

Answer

Deuterium oxide (D2O)

Answer

Evan's blue

Answer

Tritiated water (3H2O)

Question 10

The percentage of body water is greater in which group?

Accepted Answer

Males than in females

Answer

Children than in adults

Answer

Obese than in lean individuals

Answer

Old than in young individuals

Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids — MCQs

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