USMLE Physiology 3

Question 1

A patient with chronic kidney disease has metabolic acidosis, hyperphosphatemia, and hypocalcemia. What secondary complication develops due to low 1,25-dihydroxyvitamin D3?

Accepted Answer

Secondary hyperparathyroidism

Answer

CKD impairs 1-alpha hydroxylase (kidney), reducing 1,25-OH vitamin D3 production, causing hypocalcemia and secondary hyperparathyroidism with elevated PTH and renal osteodystrophy.

Question 2

Which hormone directly stimulates osteoclast activity and increases serum calcium and phosphate reabsorption in the kidney?

Accepted Answer

PTH (increases calcium, decreases phosphate reabsorption)

Answer

PTH activates osteoclasts (indirectly via osteoblasts), increases renal calcium reabsorption, decreases phosphate reabsorption, and stimulates 1-alpha hydroxylase, increasing calcitriol synthesis.

Question 3

In the autonomic nervous system, which receptor mediates bradycardia when stimulated by the vagus nerve?

Accepted Answer

Muscarinic M2 receptor

Answer

Vagal parasympathetic stimulation releases acetylcholine that binds M2 muscarinic receptors in the SA node, activating GIRK channels, hyperpolarizing the cell, and slowing heart rate.

Question 4

A patient has a Cushing reflex (hypertension, bradycardia, and irregular respirations). What does this indicate?

Accepted Answer

Increased intracranial pressure threatening brainstem herniation

Answer

The Cushing reflex is a late sign of severely elevated ICP causing brainstem ischemia; the hypertension is a compensatory response to maintain cerebral perfusion against rising ICP.

Question 5

Which change in pulmonary vascular resistance occurs during lung hyperinflation in emphysema?

Accepted Answer

Increased pulmonary vascular resistance leading to cor pulmonale

Answer

Emphysema destroys alveolar walls and pulmonary capillaries, reducing the vascular bed and increasing resistance; hypoxic vasoconstriction further increases PVR, leading to right heart failure (cor pulmonale).

USMLE Practice Test

USMLE Practice Test

USMLE Physiology 3