Electrolytes and Body Fluids Practice Questions

Q: What percentage of body weight does total body water constitute?

60%. **Explanation:** Total Body Water (TBW) is a fundamental concept in physiology, representing the sum of water in all fluid compartments. In a healthy, young adult male (the standard physiological model), water constitutes approximately **60% of the total body weight**. This percentage is slightly lower in females (approx. 50%) due to a higher proportion of subcutaneous adipose tissue, which contains very little water. **Breakdown of Options:** * **60% (Correct):** This is the standard physiological value. It follows the **"60-40-20 rule"**: 60% of body weight is TBW, 40% is Intracellular Fluid (ICF), and 20% is Extracellular Fluid (ECF). * **80% (Incorrect):** This value is characteristic of **neonates**. Newborns have a much higher water content (75–80%), which decreases rapidly during the first year of life. * **33% (Incorrect):** This value roughly corresponds to the fraction of TBW that is Extracellular Fluid (1/3 of TBW = 20% of body weight). * **25% (Incorrect):** This does not represent a standard body fluid compartment percentage. **NEET-PG High-Yield Pearls:** 1. **Tissue Variation:** Lean muscle tissue has high water content (approx. 75%), whereas adipose tissue (fat) is hydrophobic and contains only about 10% water. Therefore, obese individuals have a lower percentage of TBW compared to lean individuals. 2. **Aging:** TBW decreases with age as muscle mass declines and fat percentage increases. 3. **Calculation:** For a 70 kg man, TBW is approximately 42 Liters (70 x 0.6). 4. **ECF Sub-divisions:** The 20% ECF is further divided into Interstitial Fluid (15%) and Plasma (5%).

Q: What is the primary contributor to plasma osmolarity?

Sodium. **Explanation:** The correct answer is **Sodium (C)**. Plasma osmolarity is the measure of the concentration of solutes in the blood plasma. Under normal physiological conditions, sodium is the most abundant extracellular cation. Because it is accompanied by anions (primarily chloride and bicarbonate) to maintain electrical neutrality, sodium and its associated anions account for approximately **90-95% of the total plasma osmolarity**. The standard formula to estimate plasma osmolarity is: $2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$ This formula highlights that sodium is doubled in the calculation, reflecting its dominant role. **Analysis of Incorrect Options:** * **Urea (A):** While urea contributes to total osmolarity, it is a "permeable solute" that moves freely across cell membranes. Therefore, it does not contribute to *effective* osmolarity (tonicity). * **Glucose (B):** Glucose contributes only about 5–10 mOsm/L in a healthy individual. It becomes a significant contributor only in pathological states like Diabetes Mellitus (e.g., HHS). * **Albumin (D):** Albumin is the primary contributor to **oncotic pressure** (colloid osmotic pressure), which is essential for fluid exchange at the capillary level. However, its molar concentration is very low, making its contribution to total plasma osmolarity negligible (less than 1 mOsm/L). **Clinical Pearls for NEET-PG:** * **Normal Plasma Osmolarity:** 280–295 mOsm/L. * **Osmolar Gap:** The difference between measured and calculated osmolarity. A gap >10 mOsm/L suggests the presence of unmeasured toxins (e.g., Ethanol, Methanol, Ethylene glycol). * **Tonicity:** Sodium is the primary determinant of tonicity, which dictates the movement of water across the blood-brain barrier.

Q: Osmolality of plasma is mainly contributed by?

Sodium. **Explanation:** The osmolality of plasma is a measure of the concentration of solutes per kilogram of solvent. In the extracellular fluid (ECF), **Sodium (Na⁺)** is the most abundant cation and, along with its associated anions (primarily chloride and bicarbonate), accounts for approximately **90-95% of the total osmotic pressure** of plasma. The formula for calculated plasma osmolality is: **Calculated Osmolality = 2[Na⁺] + [Glucose]/18 + [BUN]/2.8** **Why the other options are incorrect:** * **Glucose:** While glucose is an active osmole, its contribution is relatively small in healthy individuals (approx. 5-6 mOsm/L) because its concentration is much lower than sodium. It only becomes a major contributor in pathological states like Diabetes Mellitus. * **Urea:** Urea is a "permeable" or "ineffective" osmole because it freely crosses cell membranes. While it contributes to total osmolality, it does not create an osmotic gradient across the cell membrane (tonicity). * **Uric Acid:** Present in very minute quantities in the plasma, its contribution to total plasma osmolality is negligible. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Plasma Osmolality:** 280–295 mOsm/kg. * **Osmolar Gap:** The difference between measured and calculated osmolality. A gap >10 mOsm/kg suggests the presence of unmeasured osmoles (e.g., Ethanol, Methanol, Ethylene glycol). * **Major Intracellular Osmole:** Potassium (K⁺) is the primary determinant of intracellular fluid (ICF) osmolality. * **Plasma Oncotic Pressure:** While sodium determines osmolality, **Albumin** is the primary contributor to oncotic (colloid osmotic) pressure, which keeps fluid inside the capillaries.

Q: Dehydration is more severe in infants compared to adults. What is the PRIMARY reason for this increased susceptibility?

Total body water in infants is more than in adults. ***Total body water in infants is more than in adults*** - Newborn infants have a significantly higher percentage of **Total Body Water (TBW)**, approximately **75-80%** of their body weight, compared to adults (~60%). - This large volume of water, combined with their greater **surface area-to-volume ratio** and higher **metabolic rate**, necessitates rapid fluid turnover, dramatically increasing their risk of severe dehydration. - This is the **primary physiological reason** why infants are more vulnerable to dehydration compared to adults. *Intracellular fluid (ICF) is more than extracellular fluid (ECF)* - This fluid distribution pattern is characteristic of **adults and older children**, not infants. - In young infants, the **ECF** compartment is actually greater than or nearly equal to the **ICF** compartment. - This represents the adult fluid distribution pattern, not the infant pattern. *Extracellular fluid (ECF) is more than intracellular fluid (ICF)* - While this statement is **anatomically true** for newborns (who have a disproportionately large ECF volume), it does not explain the **primary mechanism** of increased dehydration risk. - The ECF:ICF ratio changes with age, but the overall higher **total body water percentage** is the fundamental reason for severe dehydration susceptibility. - This is a characteristic of infant fluid distribution but not the main answer to why dehydration is more severe. *Extracellular fluid equals intracellular fluid* - In normal physiological states, the volume of the **ECF** compartment rarely equals the volume of the **ICF** compartment at any age. - In infants, ECF typically exceeds ICF; in adults, ICF exceeds ECF. - This statement is not accurate for either infants or adults.

Q: Which of the following is the principal constituent of serum osmolality?

Sodium. ***Sodium*** - **Sodium** is the most abundant cation in the extracellular fluid (ECF) and serum, typically present at concentrations of 135-145 mEq/L. - It is the primary determinant of **serum osmolality**, accounting for approximately 90% of the total measured osmoles, along with its accompanying anions. *Potassium* - **Potassium** is the major intracellular cation; its concentration in the serum (ECF) is very low (3.5–5.0 mEq/L). - Due to its low serum concentration, it contributes minimally and is not considered a significant factor in determining overall **serum osmolality**. *Bicarbonate* - Bicarbonate is an anion that contributes to the charge balance, but its plasma concentration (around 22–26 mEq/L) is substantially lower than that of **sodium**. - While included in the calculation of total solutes, it is not the **principal constituent** determining osmolality. *Hydrogen ions* - The concentration of **hydrogen ions** is extremely low (measured in nanomoles per liter, reflecting the pH). - Although crucial for **acid-base homeostasis**, their negligible concentration precludes any meaningful contribution to total **serum osmolality**.

Q: What is the cause of hyponatremia in diarrhea-induced hypovolemia?

Decreased aldosterone. ***Decreased aldosterone*** * In scenarios where salt loss (e.g., due to diarrhea) leads to hypovolemia, a relative or true deficiency of **aldosterone** prevents maximal sodium reabsorption in the distal tubules and collecting ducts. * This failure to maximally conserve sodium leads to **renal salt wasting**, which exacerbates the volume deficit and, when coupled with ADH-mediated water retention, results in hyponatremia. ***Decreased ADH*** * Hypovolemia (volume depletion) is the strongest non-osmotic trigger for the release of **Antidiuretic Hormone (ADH)** from the posterior pituitary, overriding low plasma osmolality [1]. * Therefore, in diarrhea-induced hypovolemia, **ADH levels are actually increased**, which drives powerful free water reabsorption in the kidney, resulting in dilutional hyponatremia [1]. ***Decreased sodium absorption from gastrointestinal tract*** * This is the primary mechanism by which diarrhea causes salt and water loss, leading to the state of **hypovolemia**. * However, the mechanism driving the *hyponatremia* (low plasma sodium concentration) involves the kidney's disproportionate reabsorption of water relative to sodium, mediated by **ADH**. ***Increased sodium absorption from kidney*** * System mechanisms like the Renin-Angiotensin-Aldosterone System (RAAS) are activated by hypovolemia to increase **sodium and water absorption** in an attempt to restore blood volume [2]. * Increased renal sodium absorption is a compensatory mechanism that works against hyponatremia; thus, it is not the cause of low plasma sodium.

Q: Which one of the following statements regarding Magnesium is NOT true?

Human adult body contains about 50 g of Magnesium. ***Human adult body contains about 50 g of Magnesium*** - The human adult body contains approximately **21-28 grams** (21000-28000 mg) of magnesium, making 50 grams an overestimation. - While magnesium is an abundant intracellular cation, 50 grams is significantly higher than the average physiological content. *It is essential for normal metabolism of calcium and potassium* - Magnesium plays a crucial role as a **cofactor** in many enzymatic reactions, including those involved in **calcium homeostasis** and **potassium transport** across cell membranes. - Adequate magnesium levels are necessary for the proper functioning of **parathyroid hormone (PTH)**, which regulates calcium and phosphate. *Daily requirement of magnesium is estimated to be about 340 mg/day for adults* - The recommended daily allowance (RDA) for magnesium in adult men is typically around **400-420 mg**, and for adult women, it's roughly **310-320 mg**. - A general estimate of 340 mg/day falls within the typical range of recommended daily intake for adults to maintain optimal health. *It is constituent of bones* - Approximately **50-60% of the body's total magnesium** is stored in the bones, contributing to their structural integrity. - It is present on the surface of **bone crystals** and plays an important role in bone metabolism and bone mineral density.

Q: The body fluid compartments of a patient were measured, showing the following ion concentrations: - Sodium (Na): $10 \mathrm{mEq} / \mathrm{L}$ - Potassium (K): $140 \mathrm{mEq} / \mathrm{L}$ - Chloride (Cl): $15 \mathrm{mEq} / \mathrm{L}$ Based on these values, which fluid compartment is being described?

ICF. ***ICF*** - The measured ion concentrations, especially **high potassium (140 mEq/L)** and **low sodium (10 mEq/L)**, are characteristic of the **intracellular fluid (ICF)**, where potassium is the primary cation and sodium is kept low by the Na+/K+-ATPase pump. - **Chloride levels (15 mEq/L)** are also significantly lower in the ICF compared to extracellular fluids. *Plasma* - Plasma typically has **high sodium (around 140 mEq/L)** and **low potassium (around 4 mEq/L)**, which contradicts the given measurements. - Chloride levels in plasma are usually much higher, around **100-105 mEq/L**. *Interstitial fluid* - Interstitial fluid has an electrolyte composition very similar to plasma, with **high sodium** and **low potassium**, differing mainly in protein content. - This composition is not consistent with the given measurements. *ECF* - The ECF (extracellular fluid), which includes both plasma and interstitial fluid, is characterized by **high sodium** and **low potassium**. - The given ion concentrations, particularly the very **high potassium** and **low sodium**, are directly opposite to the typical ECF profile.

Q: What is the normal insensible water loss?

50 mL/hour. ***50 mL/hour*** - **Insensible water loss** occurs primarily through the **skin** and **respiratory tract** and typically amounts to approximately 1200 mL per day in an adult. - Dividing 1200 mL by 24 hours yields an average of **50 mL/hour**, representing normal physiological fluid loss not readily measurable. *150 mL/hour* - This value represents a significantly **elevated rate** of insensible water loss, which would suggest a patient experiencing **fever**, **tachypnea**, or a **hot environment**. - A sustained loss at this rate would quickly lead to **dehydration** if not compensated for by increased fluid intake. *200 mL/hour* - This is an **extreme rate** of water loss, indicating a severe condition such as **severe burns** affecting a large body surface area, **heat stroke**, or profound **hyperventilation**. - Such a high rate of fluid loss would be a medical emergency requiring aggressive fluid resuscitation. *100 mL/hour* - This rate of insensible water loss is **double the normal physiological rate** and, while not as extreme as 150 or 200 mL/hour, still implies increased metabolic activity or environmental stress. - It could be seen in individuals with moderate fever, increased physical activity, or in warmer ambient temperatures, and could contribute to mild dehydration over time.

Q: A 52-year-old woman is brought to the emergency department by her husband because of weakness, abdominal pain, and a productive cough for 4 days. She also reports increased urination for the past 2 days. This morning, she had nausea and five episodes of vomiting. She has type 1 diabetes mellitus and hypertension. Current medications include insulin and lisinopril. She admits to have forgotten to take her medication in the last few days. Her temperature is 38.4°C (101.1°F), pulse is 134/min, respirations 31/min, and blood pressure is 95/61 mm Hg. Examination shows dry mucous membranes and decreased skin turgor. Abdominal examination shows diffuse tenderness with no guarding or rebound. Bowel sounds are normal. Laboratory studies show: Serum Na+ 139 mEq/L K+ 5.3 mEq/L Cl- 106 mEq/L Glucose 420 mg/dL Creatinine 1.0 mg/dL Urine Blood negative Glucose 4+ Ketones 3+ Arterial blood gas analysis on room air shows: pH 7.12 pCO2 17 mm Hg pO2 86 mm Hg HCO3- 12 mEq/L Which of the following is the most likely underlying cause of this patient's increased potassium?

Extracellular potassium shift. ***Extracellular potassium shift*** - The patient's **diabetic ketoacidosis (DKA)** leads to severe **acidosis (pH 7.12)**. In acidosis, hydrogen ions shift into cells, causing potassium to shift out of cells into the extracellular fluid to maintain electroneutrality, leading to **hyperkalemia** [2], [3]. - Additionally, the lack of insulin in DKA impairs the **Na+/K+-ATPase pump**, which normally moves potassium into cells, further contributing to extracellular potassium accumulation [2]. *Muscle cell breakdown* - While significant muscle cell breakdown (e.g., in rhabdomyolysis) can release intracellular potassium into the circulation, there is no evidence of muscle injury in this patient. - The primary driver of hyperkalemia in this context is metabolic acidosis and insulin deficiency, not muscle breakdown. *Repeated vomiting* - Repeated vomiting typically causes **hypokalemia** due to loss of gastric acid and subsequent renal potassium wasting [1]. - This patient's potassium is elevated, and while she did vomit, it is not the cause of hyperkalemia. *Increased renal potassium absorption* - Increased renal potassium absorption is not a typical physiological response to DKA and would usually be seen in conditions causing **hypovolemia with increased aldosterone** or certain **renal tubular acidosis** types, which are not the primary issue here. - In DKA, the body tries to excrete excess potassium through the kidneys, although this can be impaired by reduced renal perfusion due to dehydration [3]. *Intracellular potassium shift* - An intracellular potassium shift would lead to **hypokalemia**, not the hyperkalemia observed in this patient. - Conditions like **alkalosis** or **insulin administration** cause potassium to move into cells.

Question 1

What percentage of body weight does total body water constitute?

Accepted Answer

60%

Answer

80%

Answer

33%

Answer

25%

Question 2

What is the primary contributor to plasma osmolarity?

Accepted Answer

Sodium

Answer

Urea

Answer

Glucose

Answer

Albumin

Question 3

Osmolality of plasma is mainly contributed by?

Accepted Answer

Sodium

Answer

Glucose

Answer

Urea

Answer

Uric acid

Question 4

Dehydration is more severe in infants compared to adults. What is the PRIMARY reason for this increased susceptibility?

Accepted Answer

Total body water in infants is more than in adults

Answer

Intracellular fluid (ICF) is more than extracellular fluid (ECF)

Answer

Extracellular fluid (ECF) is more than intracellular fluid (ICF)

Answer

Extracellular fluid equals intracellular fluid

Question 5

Which of the following is the principal constituent of serum osmolality?

Accepted Answer

Sodium

Answer

Potassium

Answer

Bicarbonate

Answer

Hydrogen ions

Question 6

What is the cause of hyponatremia in diarrhea-induced hypovolemia?

Accepted Answer

Decreased aldosterone

Answer

Decreased ADH

Answer

Decreased sodium absorption from gastrointestinal tract

Answer

Increased sodium absorption from kidney

Question 7

Which one of the following statements regarding Magnesium is NOT true?

Accepted Answer

Human adult body contains about 50 g of Magnesium

Answer

It is essential for normal metabolism of calcium and potassium

Answer

Daily requirement of magnesium is estimated to be about 340 mg/day for adults

Answer

It is constituent of bones

Question 8

The body fluid compartments of a patient were measured, showing the following ion concentrations:

- Sodium (Na): $10 \mathrm{mEq} / \mathrm{L}$
- Potassium (K): $140 \mathrm{mEq} / \mathrm{L}$
- Chloride (Cl): $15 \mathrm{mEq} / \mathrm{L}$

Based on these values, which fluid compartment is being described?

Accepted Answer

ICF

Answer

Plasma

Answer

Interstitial fluid

Answer

ECF

Question 9

What is the normal insensible water loss?

Accepted Answer

50 mL/hour

Answer

150 mL/hour

Answer

200 mL/hour

Answer

100 mL/hour

Question 10

A 52-year-old woman is brought to the emergency department by her husband because of weakness, abdominal pain, and a productive cough for 4 days. She also reports increased urination for the past 2 days. This morning, she had nausea and five episodes of vomiting. She has type 1 diabetes mellitus and hypertension. Current medications include insulin and lisinopril. She admits to have forgotten to take her medication in the last few days. Her temperature is 38.4°C (101.1°F), pulse is 134/min, respirations 31/min, and blood pressure is 95/61 mm Hg. Examination shows dry mucous membranes and decreased skin turgor. Abdominal examination shows diffuse tenderness with no guarding or rebound. Bowel sounds are normal. Laboratory studies show:
Serum
Na+ 139 mEq/L
K+ 5.3 mEq/L
Cl- 106 mEq/L
Glucose 420 mg/dL
Creatinine 1.0 mg/dL
Urine
Blood negative
Glucose 4+
Ketones 3+
Arterial blood gas analysis on room air shows:
pH 7.12
pCO2 17 mm Hg
pO2 86 mm Hg
HCO3- 12 mEq/L
Which of the following is the most likely underlying cause of this patient's increased potassium?

Accepted Answer

Extracellular potassium shift

Answer

Muscle cell breakdown

Answer

Repeated vomiting

Answer

Increased renal potassium absorption

Answer

Intracellular potassium shift

Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids — MCQs

On this page

Practice by Chapter

Want unlimited practice?