USMLE Physiology 1 — Questions and Answers
Question 1: A patient with COPD has pCO2 of 55 mmHg and bicarbonate of 32 mEq/L. What is the acid-base disturbance?
- Chronic respiratory acidosis with metabolic compensation (Correct answer)
- Acute respiratory acidosis
- Metabolic alkalosis with respiratory compensation
- Mixed metabolic and respiratory acidosis
Correct answer: Chronic respiratory acidosis with metabolic compensation
Chronic respiratory acidosis shows elevated pCO2 (hypoventilation) with appropriately elevated bicarbonate (renal compensation of 3.5 mEq/L per 10 mmHg rise in pCO2), distinguishing chronic from acute forms.
Question 2: Starling forces govern fluid movement across capillary membranes. Which force promotes fluid reabsorption from the interstitium into capillaries?
- Plasma oncotic pressure (COP) (Correct answer)
- Capillary hydrostatic pressure
- Interstitial oncotic pressure
- Lymphatic obstruction
Correct answer: Plasma oncotic pressure (COP)
Plasma oncotic pressure (colloid osmotic pressure from plasma proteins, mainly albumin) draws fluid from the interstitium back into capillaries, counteracting capillary hydrostatic pressure.
Question 3: The Frank-Starling mechanism states that increased preload leads to increased stroke volume. What is the molecular basis for this?
- Increased overlap of actin and myosin leads to more cross-bridge formation (Correct answer)
- Increased calcium influx
- Decreased afterload
- Increased heart rate
Correct answer: Increased overlap of actin and myosin leads to more cross-bridge formation
Stretching myocardial fibers (increased preload) increases actin-myosin overlap and sensitizes troponin C to calcium, increasing cross-bridge formation and contractile force.
Question 4: During exercise, which physiological change causes oxygen delivery to be increased to working muscles?
- Rightward shift of the oxygen-hemoglobin dissociation curve (Correct answer)
- Leftward shift of the oxygen-hemoglobin dissociation curve
- Increased hemoglobin oxygen affinity
- Decreased 2,3-DPG
Correct answer: Rightward shift of the oxygen-hemoglobin dissociation curve
Exercise increases CO2, temperature, H+, and 2,3-DPG in working muscles, shifting the O2-Hb dissociation curve rightward (Bohr effect), decreasing hemoglobin affinity and increasing O2 delivery.
Question 5: A patient has syndrome of inappropriate antidiuretic hormone (SIADH). What is the expected urine osmolality?
- High urine osmolality (>100 mOsm/kg) (Correct answer)
- Low urine osmolality (<50 mOsm/kg)
- Isotonic urine
- Variable depending on fluid intake
Correct answer: High urine osmolality (>100 mOsm/kg)
In SIADH, excess ADH causes inappropriate water retention and concentrated urine (>100 mOsm/kg, often >300 mOsm/kg) despite low serum osmolality.
Question 6: Which segment of the nephron is responsible for the countercurrent multiplication that generates the medullary osmotic gradient?
- Loop of Henle (Correct answer)
- Proximal convoluted tubule
- Distal convoluted tubule
- Collecting duct
Correct answer: Loop of Henle
The loop of Henle creates the medullary osmotic gradient through countercurrent multiplication: the descending limb is permeable to water (concentrating tubular fluid) while the ascending limb actively pumps NaCl out.
USMLE Physiology 1
A patient with COPD has pCO2 of 55 mmHg and bicarbonate of 32 mEq/L.
What is the acid-base disturbance?