USMLE Physiology

Question 1

A patient with COPD has pCO2 of 55 mmHg and bicarbonate of 32 mEq/L. What is the acid-base disturbance?

Accepted Answer

Chronic respiratory acidosis with metabolic compensation

Answer

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?

Accepted Answer

Plasma oncotic pressure (COP)

Answer

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?

Accepted Answer

Increased overlap of actin and myosin leads to more cross-bridge formation

Answer

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?

Accepted Answer

Rightward shift of the oxygen-hemoglobin dissociation curve

Answer

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?

Accepted Answer

High urine osmolality (>100 mOsm/kg)

Answer

In SIADH, excess ADH causes inappropriate water retention and concentrated urine (>100 mOsm/kg, often >300 mOsm/kg) despite low serum osmolality.

USMLE Practice Test

USMLE Practice Test

USMLE Physiology