Thermoregulation and Body Fluids Practice Questions
20 free Thermoregulation and Body Fluids practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
A patient with heat stroke presents with confusion and a core body temperature of 41.5°C (106.7°F). Which physiological mechanism is primarily overwhelmed in classic heat stroke?
- A Cutaneous vasoconstriction response
- B Evaporative heat loss via sweating
- C Shivering-based thermogenesis
- D Nonshivering thermogenesis pathway
Correct answer: Evaporative heat loss via sweating
Heat stroke occurs when evaporative heat loss via sweating becomes insufficient to dissipate heat. This leads to uncontrolled rise in core temperature and CNS dysfunction.
A patient in a cold environment begins shivering. Which area of the hypothalamus primarily mediates this response?
- A Anterior hypothalamus
- B Posterior hypothalamus
- C Lateral hypothalamus
- D Ventromedial hypothalamus
Correct answer: Posterior hypothalamus
The posterior hypothalamus activates heat-producing mechanisms such as shivering. The anterior hypothalamus regulates heat dissipation.
A marathon runner becomes dehydrated. Which fluid compartment will lose the greatest volume first?
- A Intracellular fluid
- B Interstitial fluid
- C Plasma volume
- D Transcellular fluid
Correct answer: Plasma volume
Plasma volume decreases first during dehydration because water is lost from the extracellular compartment. Subsequent osmotic shifts affect other compartments.
A patient is given 2 liters of isotonic saline. Which compartment increases the most?
- A Intracellular fluid
- B Extracellular fluid
- C Plasma only
- D Interstitial fluid only
Correct answer: Extracellular fluid
Isotonic saline expands the extracellular fluid compartment because it does not cross cell membranes. Both plasma and interstitial fluid increase.
A patient with SIADH develops hyponatremia. Which change is most expected in intracellular fluid volume?
- A Decreased ICF volume
- B Increased ICF volume
- C No change in ICF volume
- D Shift of sodium into cells
Correct answer: Increased ICF volume
Hyponatremia lowers plasma osmolality, causing water to shift into cells. This increases intracellular volume.
A person exposed to heat undergoes cutaneous vasodilation. Which physiological advantage does this provide?
- A Decreases evaporative heat loss
- B Increases conductive heat gain from surroundings
- C Increases convective and radiative heat loss
- D Reduces core body dehydration risk
Correct answer: Increases convective and radiative heat loss
Vasodilation increases blood flow to the skin, enhancing convective and radiative heat loss. This helps regulate body temperature.
A patient develops a fever. Which mediator increases the hypothalamic set point?
- A IL-1
- B TNF-α
- C Prostaglandin E2
- D Interferon-γ
Correct answer: Prostaglandin E2
PGE2 increases the hypothalamic set point during fever. IL-1 and TNF-α stimulate PGE2 production.
A patient with heat exhaustion collapses after prolonged sweating. Which abnormality is most likely?
- A Hypernatremia
- B Hyponatremia
- C Metabolic alkalosis
- D Increased plasma osmolality
Correct answer: Hyponatremia
Prolonged sweating leads to sodium loss, which can cause hyponatremia. Heat exhaustion occurs without CNS dysfunction.
A patient undergoing therapeutic hypothermia begins shivering. Giving sedatives stops shivering by reducing activity of which tissue?
- A Brown adipose tissue
- B Skeletal muscle
- C Smooth muscle
- D Cardiac muscle
Correct answer: Skeletal muscle
Shivering thermogenesis occurs in skeletal muscle. Sedatives suppress this activity to maintain hypothermia.
Which fluid is the primary determinant of extracellular osmolality?
- A Potassium
- B Sodium
- C Calcium
- D Glucose
Correct answer: Sodium
Sodium is the major extracellular cation and primary determinant of plasma osmolality. Changes in sodium concentration affect water balance.
A patient is given hypertonic saline. What is the immediate effect on cell volume?
- A Cells swell
- B Cells shrink
- C No change
- D Cells undergo lysis
Correct answer: Cells shrink
Hypertonic saline increases extracellular osmolality, drawing water out of cells. This causes cells to shrink.
A patient with diabetes insipidus has decreased ADH activity. Which change best describes their urine?
- A High volume, low osmolality
- B Low volume, high osmolality
- C Low volume, low osmolality
- D Normal volume, normal osmolality
Correct answer: High volume, low osmolality
ADH deficiency prevents water reabsorption, producing large volumes of dilute urine. Plasma osmolality increases as a result.
A mountaineer exposed to cold develops frostbite. Which mechanism contributes most to tissue damage?
- A Increased metabolic heat production
- B Vasodilation leading to fluid leakage
- C Prolonged vasoconstriction reducing flow
- D Accelerated sweating with evaporation
Correct answer: Prolonged vasoconstriction reducing flow
Cold exposure triggers persistent vasoconstriction, reducing perfusion and leading to tissue ischemia. This causes frostbite injury.
An infant has had several days of osmotic diarrhea with very little free-water intake. Which electrolyte abnormality is most likely?
- A Hyponatremia
- B Hypernatremia
- C Hypercalcemia
- D Hyperkalemia
Correct answer: Hypernatremia
In osmotic diarrhea the stool is hypotonic, so free water is lost in excess of sodium. When that water is not replaced, plasma sodium and osmolality rise and hypernatremic dehydration develops. Diarrhoeal potassium loss produces hypokalemia rather than hyperkalemia, and serum calcium is not driven up by fluid loss of this kind.
Which mechanism is the primary means of heat loss during exercise in a hot, dry climate?
- A Conduction
- B Convection
- C Evaporation
- D Radiation
Correct answer: Evaporation
Evaporation of sweat is the main method of heat loss when ambient temperature exceeds skin temperature. Other methods become ineffective.
A patient is immersed in cold water and develops nonshivering thermogenesis. Which tissue mediates this response?
- A White adipose tissue
- B Cardiac muscle
- C Brown adipose tissue
- D Skeletal muscle
Correct answer: Brown adipose tissue
Brown fat contains uncoupling proteins that generate heat without shivering. This mechanism is prominent in infants.
A patient receives 1 liter of D5W (5% dextrose in water). After metabolism of glucose, the fluid distributes primarily into which compartment?
- A Intracellular fluid
- B Extracellular fluid
- C Plasma volume only
- D Interstitial fluid only
Correct answer: Intracellular fluid
D5W becomes free water after glucose metabolism and distributes across all compartments proportionally. Most water enters the intracellular space.
A patient becomes hyperthermic after taking MDMA. Which mechanism is most responsible for the excessive heat production?
- A Excessive sweating response
- B Sympathetic overactivation
- C Peripheral vasodilation
- D Reduced metabolic rate
Correct answer: Sympathetic overactivation
MDMA causes massive sympathetic activation and hypermetabolism, greatly increasing heat production. This can overwhelm heat loss mechanisms.
A patient with severe hyponatremia has seizures. Which cellular change contributes most to neuronal dysfunction?
- A Neuronal shrinkage
- B Neuronal swelling
- C Loss of potassium from neurons
- D Increased calcium influx
Correct answer: Neuronal swelling
Hyponatremia decreases extracellular osmolality, causing water to move into neurons. Cytotoxic swelling leads to neurologic symptoms.
A researcher measures core temperature via the preoptic area of the hypothalamus. Which physiological process does this area regulate?
- A Heat conservation mechanisms
- B Heat dissipation mechanisms
- C Shivering reflex
- D Brown fat activation
Correct answer: Heat dissipation mechanisms
The preoptic area (anterior hypothalamus) regulates heat loss via vasodilation and sweating. It maintains core temperature during heat exposure.