Question 1

Elena experienced flaccid paralysis in her left leg after exposure to a neurotoxin. Electrical stimulation of her motor neuron produces no end-plate potential (EPP) and no increase in intracellular Ca$$^{2+} in $$the muscle fiber. Which of the following best explains these findings?

Accepted Answer

Botulinum toxin exposure, which prevents acetylcholine (ACh) release from the motor neuron, resulting in no EPP and no muscle activation.

Question 2

Direct injection of acetylcholine (ACh) into Elena’s neuromuscular junction results in a normal end-plate potential, a significant increase in intracellular Ca$$^{2+}$$, but no force production. Which of the following is the most likely explanation?

Accepted Answer

A defect in the contractile proteins (e.g., actin or myosin) preventing force generation despite normal excitation-contraction coupling.

Question 3

Which of the following defects could explain failure to produce force despite a normal increase in intracellular Ca$$^{2+} in $$skeletal muscle?

Accepted Answer

A defect in myosin ATPase activity, preventing cross-bridge cycling and force production.

Question 4

After several weeks of therapy, Elena’s muscle produces a small amount of force with direct ACh application, and force increases modestly with higher stimulation frequency, but remains far below normal. Which of the following is the most likely explanation?

Accepted Answer

Partial recovery of contractile protein function, but persistent structural damage limits maximal force.

Question 5

Based on all findings—no EPP with motor neuron stimulation, normal EPP and Ca$$^{2+}$$ with direct ACh, low force production, and partial recovery over time—which conclusion is best supported?

Accepted Answer

Initial presynaptic failure (e.g., botulinum toxin) with subsequent partial recovery and residual contractile apparatus damage.

Question 6

Elena’s ECG shows P waves at regular intervals, a prolonged PR interval, some P waves not followed by QRS complexes, and normal QRS duration. Which of the following is the most likely diagnosis?

Accepted Answer

Second-degree AV block (Mobitz type I or II), indicating intermittent failure of conduction through the AV node.

Question 7

After administration of a drug that increases sympathetic stimulation (β$$_1-$$adrenergic effect), Elena’s heart rate increases, PR interval shortens, but some P waves are still not followed by QRS complexes. What does this suggest?

Accepted Answer

Sympathetic stimulation improves AV conduction but does not fully resolve the AV block.

Question 8

During a cardiac cycle in which a P wave is not followed by a QRS complex, which mechanical event is most likely occurring?

Accepted Answer

The atria contract, but the ventricles do not contract, resulting in no ventricular systole or ejection.

Question 9

During a cardiac cycle with a QRS complex present, you measure ventricular pressure greater than atrial pressure but less than aortic pressure, and ventricular volume is decreasing. Which phase of the cardiac cycle does this represent?

Accepted Answer

Ventricular ejection phase, when the semilunar valves are open and blood is leaving the ventricle.

Question 10

During exercise, Elena’s heart rate increases, but some P waves are still not followed by QRS complexes, and her stroke volume is lower than expected. Which of the following best explains these findings?

Accepted Answer

Persistent AV conduction block limits ventricular filling and output, reducing stroke volume despite increased heart rate.

Question 11

Which of the following changes would most effectively increase blood flow to Elena’s active skeletal muscles during exercise?

Accepted Answer

Decreased arteriolar resistance (vasodilation) in skeletal muscle, increasing perfusion according to $$Q = \frac{\Delta P}{R}.$$

Question 12

During exercise, Elena’s arterioles supplying skeletal muscle show less vasodilation than expected, and capillary hydrostatic pressure is lower than normal. What is the most likely consequence?

Accepted Answer

Reduced muscle perfusion and impaired delivery of oxygen and nutrients during exercise.

Question 13

During exercise, Elena’s capillary hydrostatic pressure is lower than expected, interstitial hydrostatic pressure is unchanged, capillary oncotic pressure is slightly increased, and interstitial oncotic pressure is unchanged. According to Starling’s equation, what is the predicted effect on net filtration?

Accepted Answer

Net filtration out of the capillary will decrease, potentially reducing tissue fluid delivery.

Question 14

Despite increased heart rate during exercise, Elena’s mean arterial pressure (MAP) does not rise as expected. Which of the following best explains this finding?

Accepted Answer

Inadequate increase in cardiac output or excessive vasodilation prevents MAP from rising, as described by $$MAP = CO 	imes TPR.$$

Question 15

Elena becomes lightheaded during exercise, and you record a transient drop in MAP. Which of the following physiological responses would the body initiate to restore MAP?

Accepted Answer

Activation of the baroreceptor reflex, increasing sympathetic outflow to the heart and vessels, raising heart rate, contractility, and vasoconstriction.

Question 16

After donating 500 mL of blood, Elena’s heart rate is significantly higher during exercise at the same workload. Which of the following best explains this physiological response?

Accepted Answer

Reduced blood volume decreases stroke volume, so heart rate increases to maintain cardiac output ($$CO = HR 	imes SV).$$

Question 17

Despite maintaining the same running pace after blood donation, Elena feels more fatigued and tires quickly. Which of the following is the primary physiological limitation?

Accepted Answer

Reduced oxygen-carrying capacity due to lower hemoglobin concentration limits aerobic metabolism.

Step-by-Step Guidance for Neuromuscular, Cardiac, and Vascular Physiology Case Study

Study Guide - Practice Questions

Study Guide - Flashcards