PCAT Physiology 5 — Questions and Answers
Question 1: In skeletal muscle, calcium released from the sarcoplasmic reticulum binds to which protein to initiate contraction?
- Myosin heavy chain
- Tropomyosin directly
- Troponin C (Correct answer)
- Actin G-subunit
Correct answer: Troponin C
Ca²⁺ binds troponin C, causing a conformational change that moves tropomyosin away from actin's myosin-binding sites, allowing cross-bridge cycling.
Question 2: Which hormone acts on bone to INCREASE osteoclast activity and raise serum calcium?
- Calcitonin
- Parathyroid hormone (PTH) (Correct answer)
- Calcitriol (1,25-OH₂ vitamin D)
- Estrogen
Correct answer: Parathyroid hormone (PTH)
PTH stimulates osteoclast-mediated bone resorption (via RANKL on osteoblasts), releasing calcium and phosphate into the blood to correct hypocalcemia.
Question 3: The cerebellum contributes to motor coordination primarily by:
- Initiating voluntary movements via pyramidal tracts
- Comparing intended movement to actual movement and correcting errors (Correct answer)
- Processing pain signals from the periphery
- Controlling circadian rhythm and sleep cycles
Correct answer: Comparing intended movement to actual movement and correcting errors
The cerebellum receives efference copies of motor commands and proprioceptive feedback, computing error signals to fine-tune movement smoothness and timing.
Question 4: In the proximal convoluted tubule, approximately what percentage of filtered sodium is reabsorbed?
- 25%
- 50%
- 67% (Correct answer)
- 90%
Correct answer: 67%
About two-thirds (67%) of filtered Na⁺ is reabsorbed in the PCT via Na⁺/H⁺ exchange and cotransport, making it the major site of bulk tubular reabsorption.
Question 5: Which change would increase airway resistance the MOST according to Poiseuille's law?
- Doubling airway length
- Halving airway radius (Correct answer)
- Doubling gas viscosity
- Quadrupling driving pressure
Correct answer: Halving airway radius
Resistance is inversely proportional to radius to the fourth power (R ∝ 1/r⁴), so halving the radius increases resistance 16-fold—far more than other changes.
Question 6: The sympathetic nervous system's effect on the pupil is:
- Miosis via stimulation of the sphincter pupillae
- Mydriasis via stimulation of the dilator pupillae (Correct answer)
- No effect on pupil size
- Constriction via muscarinic receptor activation
Correct answer: Mydriasis via stimulation of the dilator pupillae
Sympathetic α₁ adrenergic stimulation contracts the radial dilator pupillae muscle, widening the pupil (mydriasis) to maximize light entry under threat.
Question 7: Chronic hyperventilation leads to which acid-base disturbance?
- Metabolic alkalosis with compensatory hypoventilation
- Respiratory alkalosis due to excess CO₂ elimination (Correct answer)
- Respiratory acidosis from CO₂ accumulation
- Metabolic acidosis from lactic acid buildup
Correct answer: Respiratory alkalosis due to excess CO₂ elimination
Excess CO₂ washout from hyperventilation reduces PCO₂, shifting the carbonic acid equilibrium and raising pH—producing primary respiratory alkalosis.
PCAT Physiology 5
In skeletal muscle, calcium released from the sarcoplasmic reticulum binds to which protein to initiate contraction?