NBME Practice Questions

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Question 1 of 1

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443672

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Excessive vomiting in the short term can lead to hypokalemia with no change in intracellular [K+]. One possible result of this is:

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Explanation

Lower potassium levels in the extracellular space cause hyperpolarization of the resting membrane potential. This hyperpolarization is caused by the effect of the altered potassium gradient on resting membrane potential as defined by the Goldman equation. As a result, a greater than normal stimulus is required for depolarization of the membrane to initiate an action potential

443674

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The stimulation of nerve endings in the Golgi tendon organs leads directly to

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443676

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A new drug is developed that proposes to decrease the rate of action potentials by increasing the amount of inhibitory postsynaptic potentials a cell experiences. Which of the following is the most likely mechanism of action for such a drug

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Explanation

N/A

443681

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Digitalis, a drug used in the treatment of heart failure, produces its therapeutic effect by inhibition of the Na+/K+-ATPase pump. Which of the following cellular effects is this drug most likely to have?

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Explanation

The inhibition of Na+/K+-ATPase induces a rise in sodium concentration inside cells. This sodium increase induces in its turn an increase in the intracellular calcium concentration, via the sodium-calcium exchanger. The sodium-calcium exchanger is particularly active in myocardium and in smooth vascular muscles. The rise in intracellular calcium increases the force of contraction of the heart and smooth vascular muscles. The inhibition of Na+/K+-ATPase reduces cellular polarization (depolarizing effect).

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Which of the following events is most closely related to the major mechanism of repolarization of excitable cells?

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Explanation

Repolarization of the action potential is due to efflux of K+ from the cell

443685

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A 36-year-old woman is evaluated because of a two-week history of fatigue and occasional blurry vision. She works as a third-grade teacher and reports to generally feeling fine while at work. But when she returns home from work, she fatigues easily and may experience blurred vision. Neurologic examination shows normal strength and reflexes, but when asked to quickly look upward from a downward position after initial over-elevation, her eyelids droop. This finding disappears after the administration of edrophonium. Which of the following best explains the process by which this patient’s disease produces symptoms of muscle weakness?

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Explanation

The patient has myasthenia gravis. Giving an acetylcholinesterase inhibitor leaves more acetylcholine in the synaptic cleft to improve symptoms.

443688

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A new drug is developed that prevents the demyelinization occurring in the progress of multiple sclerosis. The drug protects the cells responsible for the synthesis and maintenance of myelin in the central nervous system. These cells are most likely which of the following?

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Explanation

Oligodendrocytes are the myelinating cells of the CNS. Schwann cells are the myelinating cells of the PNS.

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The release of epinephrine from the chromaffin granules of the adrenal medulla into the bloodstream in response to neural stimulation is mediated by which of the following?

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Explanation

Acetylcholine released by preganglionic sympathetic fibers of these nerves acts on nicotinic acetylcholine receptors, causing cell depolarization and an influx of calcium through voltage-gated calcium channels. Calcium triggers the exocytosis of chromaffin granules and, thus, the release of adrenaline (and noradrenaline) into the bloodstream.

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An experimental drug is being tested as a potential therapeutic treatment for asthma. Studies have shown that this drug induces the relaxation of cultured porcine tracheal smooth cells pre-contracted with acetylcholine. Which of the following mechanisms of action is most likely to induce this effect?

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Explanation

Stimulation of AC or GC induces smooth muscle relaxation. AC and C stimulate cAMP and cGMP-dependent kinases that phosphorylate enzymes that remove Ca2+ from the cytosol leading to decreased contraction. Decreasing K+ permeability or increased Na+ permeability leads to depolarization resulting in contraction. Inhibition of the Ca2+-ATPase favors contraction. No Tn in smooth muscle.

443694

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Drug X applied to a nerve axon decreases the duration of the action potential without affecting the resting potential or peak amplitude of the action potential. Which of the following is the most likely mechanism of action of Drug X?

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445748

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An 82-year-old woman is brought to the emergency department complaining of nausea, vomiting, muscle cramps, and generalized weakness. Laboratory analysis reveals significant hyperkalemia. Elevations of extracellular potassium ion concentration will have which of the following effects on cell membranes?

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445763

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A 48-year-old man has hepatic cancer that is unresponsive to standard therapy. He enrolls in a clinical study of a novel chemotherapeutic agent that, as a side effect, blocks kinesin, a component of the cellular microtubular transport system. One week later, he develops skeletal muscle weakness. An alteration in which of the following components of the neuromuscular junction is the most likely cause of the muscle weakness?

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443718

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A 19-year-old woman with a history of diplopia and paresthesia is diagnosed with multiple sclerosis (MS). Immersion of an affected limb in a cold bath restores nerve conduction in many MS patients. The explanation often cited for this effect is that cold increases the duration of the action potential. Which of the following best explains why increasing the duration of the action potential can restore nerve conduction in patients with MS?

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Explanation

Cold delays inactivation of Na+ channels allowing more Na+ to enter the cell. Increased duration does not equal increase membrane potential. No change to capacitance or refractory period. Duration of the AP depends on Na+ not K+.

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Which of the following statements is incorrect?

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Explanation

Because the ratio of Na+ to K+channels is low in adult astrocytes, these cells are not capable of regenerative electrical responses such as the action potential.

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In the CNS a single oligodendrocyte myelinates a single axon.

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Explanation

In the PNS, a single Schwann cell provides a single myelin segment to a single axon of a myelinated nerve (see Fig. 11-13B). This situation stands in contrast to that in the CNS, where one oligodendrocyte myelinates many axons.

NBME Practice Questions

Question 1 of 1

Time Left

Excessive vomiting in the short term can lead to hypokalemia with no change in intracellular [K+]. One possible result of this is:

Select the correct answer(s).

(0/0)

Explanation

The stimulation of nerve endings in the Golgi tendon organs leads directly to

Select the correct answer(s).

(0/0)

A new drug is developed that proposes to decrease the rate of action potentials by increasing the amount of inhibitory postsynaptic potentials a cell experiences. Which of the following is the most likely mechanism of action for such a drug

Select the correct answer(s).

(0/0)

Explanation

Digitalis, a drug used in the treatment of heart failure, produces its therapeutic effect by inhibition of the Na+/K+-ATPase pump. Which of the following cellular effects is this drug most likely to have?

Select the correct answer(s).

(0/0)

Explanation

Which of the following events is most closely related to the major mechanism of repolarization of excitable cells?

Select the correct answer(s).

(0/0)

Explanation

Select the correct answer(s).

(0/0)

Explanation

A new drug is developed that prevents the demyelinization occurring in the progress of multiple sclerosis. The drug protects the cells responsible for the synthesis and maintenance of myelin in the central nervous system. These cells are most likely which of the following?

Select the correct answer(s).

(0/0)

Explanation

The release of epinephrine from the chromaffin granules of the adrenal medulla into the bloodstream in response to neural stimulation is mediated by which of the following?

Select the correct answer(s).

(0/0)

Explanation

Select the correct answer(s).

(0/0)

Explanation

Drug X applied to a nerve axon decreases the duration of the action potential without affecting the resting potential or peak amplitude of the action potential. Which of the following is the most likely mechanism of action of Drug X?

Select the correct answer(s).

(0/0)

Select the correct answer(s).

(0/0)

Select the correct answer(s).

(0/0)

Select the correct answer(s).

(0/0)

Explanation

Which of the following statements is incorrect?

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Explanation

In the CNS a single oligodendrocyte myelinates a single axon.

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Explanation

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