Physiology for USMLE Step 1

A healthy volunteer is given a drug that selectively blocks the Na⁺/K⁺/2Cl⁻ cotransporter in the thick ascending limb of the loop of Henle. Which set of changes would you expect?

1. Metabolic acidosis with a normal anion gap Wrong. Normal-gap metabolic acidosis follows blocking carbonic anhydrase with acetazolamide. Loop diuretics instead cause a contraction ALKALOSIS, where the blood becomes too basic from fluid loss.
2. Increased urinary calcium excretion and reduced ability to concentrate urine correct Correct. The Na+/K+/2Cl- cotransporter creates a positive electrical charge inside the tubule that pushes calcium and magnesium to be reabsorbed between cells. Blocking it causes more calcium and magnesium to be lost in the urine (calciuria and magnesuria). It also breaks down the concentration gradient in the kidney's medulla, so urine cannot be concentrated as much as normal.
3. Hyperkalemia from reduced distal sodium delivery Wrong, this is the opposite of what happens. More sodium reaching the distal tubule drives more potassium loss, so loop diuretics cause LOW potassium (hypokalemia), not high.
4. Increased medullary interstitial osmolality Wrong, again the opposite. The cotransporter is what builds up the concentration gradient in the kidney's medulla, so blocking it LOWERS medullary osmolality rather than raising it.
5. Decreased urinary calcium excretion and hypercalcemia Wrong. Lower urinary calcium and high blood calcium describe the THIAZIDE effect, which works on a different part of the kidney (the distal convoluted tubule). This is why thiazide drugs, not loop diuretics, are used in patients who keep forming calcium kidney stones.

The point: Loop diuretics cause calcium loss; thiazides keep calcium in the body. This is why loop diuretics (with IV fluids) are used to treat high calcium, and thiazides are used to prevent calcium kidney stones. Both drug types cause low potassium with a metabolic alkalosis. Loop diuretics can also damage hearing (ototoxicity), especially when combined with aminoglycoside antibiotics.

Source: USMLE Content Outline — renal physiology and pharmacology NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator

A patient is given a pure alpha-1 agonist. Mean arterial pressure rises from 85 to 115 mm Hg and heart rate falls from 78 to 58 beats per minute. The patient is not taking any other drug. What mediates the fall in heart rate?

1. Beta-2 mediated vasodilation in skeletal muscle Wrong. A pure alpha-1 agonist does not act on beta-2 receptors, and blood vessel widening (vasodilation) would lower blood pressure, not raise it as seen here.
2. Increased circulating atrial natriuretic peptide Wrong. Atrial natriuretic peptide (ANP) does rise when the heart's upper chambers stretch, but it works over minutes to hours on fluid volume, not on the fast, beat-to-beat heart rate change seen here.
3. Baroreceptor-mediated increase in vagal tone correct Correct. The stretch receptors in the carotid sinus and aortic arch (baroreceptors) fire more often when blood pressure rises. This increases parasympathetic (vagal) signals to the heart's natural pacemaker (SA node) and reduces sympathetic tone. This reflex slowing of the heart rate (reflex bradycardia) is the giveaway sign of a pure vasoconstrictor drug.
4. Reduced preload from venodilation Wrong. Alpha-1 stimulation causes veins to tighten (venoconstriction), which INCREASES the amount of blood returning to the heart (preload), not reduces it.
5. Direct alpha-1 blockade of the SA node Wrong. The drug is an agonist, meaning it activates the receptor rather than blocking it, and the heart's pacemaker (SA node) is controlled mainly by beta-1 receptors, not alpha-1 receptors.

The point: Reflex slowing of the heart (bradycardia) along with a rise in blood pressure points to a pure alpha agonist drug, like phenylephrine. Reflex speeding of the heart (tachycardia) along with a drop in blood pressure points to a vasodilator drug. If a pressor drug raises blood pressure WITHOUT causing reflex bradycardia, suspect the drug also has beta-1 activity.

Source: USMLE Content Outline — cardiovascular physiology NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator

A healthy 24-year-old is studied upright at rest. Compared with the apex of the lung, the base has higher ventilation and higher perfusion, but a LOWER ventilation/perfusion ratio. Which set of alveolar gas values best describes the apex relative to the base?

1. Higher PO2 and lower PCO2 at the apex correct Correct. Gravity reduces blood flow (perfusion) at the top of the lung (apex) more than it reduces air flow (ventilation), so the V/Q ratio at the apex is high, closer to dead space, meaning air reaches these alveoli but there is little blood flow to exchange with. The air there ends up close to what we breathe in: high oxygen (PO2), low carbon dioxide (PCO2). This is also why reactivated tuberculosis tends to settle in the apex.
2. Higher PO2 and higher PCO2 at the apex Wrong. This does not make physiological sense: in a high V/Q region, oxygen goes up as carbon dioxide gets washed out, so the two move in opposite directions, not the same direction.
3. Lower PO2 and higher PCO2 at the apex Wrong. Lower PO2 and higher PCO2 describe the base of the lung, where the low V/Q ratio makes the air in the alveoli look more like the oxygen-poor blood returning to the heart (mixed venous blood).
4. Identical PO2 and PCO2 throughout the upright lung Wrong. Identical gas levels throughout the lung are only true in zero gravity, or lying flat along that particular axis. When standing upright, the difference between apex and base is substantial.
5. Lower PO2 and lower PCO2 at the apex Wrong. Lower PO2 and lower PCO2 together describe a shunt, where blood bypasses ventilated lung, not a high V/Q region like the apex.

The point: Apex of the lung: high V/Q ratio, high PO2, low PCO2, and this is where TB tends to settle. Centriacinar emphysema is not found at the apex, but panacinar emphysema from alpha-1 antitrypsin deficiency is found at the BASE. Base of the lung: low V/Q ratio, with more ventilation and much more blood flow (perfusion) in absolute terms.

Source: USMLE Content Outline — respiratory physiology NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator