CRNI Exam Fluid and Electrolyte Balance 2 — Questions and Answers
Question 1: Which electrolyte is the primary determinant of extracellular fluid osmolality?
- Potassium
- Calcium
- Sodium (Correct answer)
- Magnesium
Correct answer: Sodium
Sodium is the dominant cation in extracellular fluid and accounts for approximately 90% of extracellular osmolality, making it the principal regulator of ECF volume and tonicity.
Question 2: Antidiuretic hormone (ADH) is released in response to which of the following stimuli?
- Decreased serum osmolality
- Increased serum osmolality (Correct answer)
- Decreased serum potassium
- Increased urine specific gravity
Correct answer: Increased serum osmolality
ADH is secreted by the posterior pituitary when osmoreceptors detect a rise in plasma osmolality. ADH promotes water reabsorption in the renal collecting ducts to dilute the plasma and restore normal osmolality.
Question 3: A patient has a serum potassium of 2.8 mEq/L. Which clinical finding would the nurse most expect to assess?
- Peaked T waves on ECG
- Muscle weakness and fatigue (Correct answer)
- Bradycardia with heart block
- Hyperreflexia
Correct answer: Muscle weakness and fatigue
Hypokalemia (serum K⁺ < 3.5 mEq/L) impairs neuromuscular conduction, leading to skeletal muscle weakness and fatigue. Peaked T waves and bradycardia are associated with hyperkalemia, not hypokalemia.
Question 4: Which fluid compartment contains the largest proportion of total body water in an adult?
- Intravascular fluid
- Interstitial fluid
- Intracellular fluid (Correct answer)
- Transcellular fluid
Correct answer: Intracellular fluid
Intracellular fluid (ICF) accounts for approximately 40% of body weight, or about two-thirds of total body water — far exceeding any other single compartment. Extracellular fluid (interstitial + intravascular) makes up the remaining one-third.
Question 5: Which mechanism describes the movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration?
- Diffusion
- Active transport
- Filtration
- Osmosis (Correct answer)
Correct answer: Osmosis
Osmosis is the passive movement of water across a semi-permeable membrane down its own concentration gradient — from a region of lower solute (higher water) concentration to higher solute (lower water) concentration — until equilibrium is reached.
Question 6: A patient receiving rapid, large-volume infusion of 0.9% normal saline is at greatest risk for which electrolyte imbalance?
- Hypernatremia
- Hyperchloremic metabolic acidosis (Correct answer)
- Hypokalemia
- Hypercalcemia
Correct answer: Hyperchloremic metabolic acidosis
Normal saline contains 154 mEq/L of chloride, which exceeds plasma chloride levels (~103 mEq/L). Large infusions deliver excess chloride, suppressing bicarbonate reabsorption and producing a dilutional hyperchloremic metabolic acidosis.
CRNI Exam Fluid and Electrolyte Balance 2
Which electrolyte is the primary determinant of extracellular fluid osmolality?