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Free STEP1 Exam Braindumps (page: 60)

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PDF with 845 Questions

Altered plasma renin levels can occur in both normal and pathologic conditions. Which of the following states is associated with a decrease in plasma renin levels?

  1. heart failure
  2. primary aldosteronism
  3. renal artery stenosis
  4. salt restriction
  5. upright posture

Answer(s): B

Explanation:

Most patients with primary aldosteronism (Conn syndrome) have an adrenal adenoma. The increased plasma aldosterone concentration leads to increased renal Na+ reabsorption, which results in plasma volume expansion. The increase in plasma volume suppresses renin release from the juxtaglomerular apparatus and these patients usually have low plasma renin levels. Secondary aldosteronism is due to elevated renin levels and may be caused by heart failure (choice A) or renal artery stenosis (choice C). Salt restriction (choice D) and upright posture (choice E) decrease renal perfusion pressure and therefore increase renin release from the juxtaglomerular apparatus.




Below figure shows the change in the membrane potential of a postsynaptic neuron caused by addition, at the arrow, of a neurotransmitter. The resting membrane potential of this cell is -80 mV. Relevant equilibrium potentials for +, , +, , and are, respectively, +120 mV, +60 mV, 0 mV, -80 mV, and -90 mV. This neurotransmitter likely increases the conductance of which of the following ions?

  1. A
  2. B
  3. C
  4. D
  5. E

Answer(s): C

Explanation:

Increase in the conductance of an ion, the equilibrium potential of which is negative with respect to the cell's resting membrane potential, will cause the membrane potential to hyperpolarize (to become more negative). Increased permeability would thus cause hyperpolarization, since its equilibrium potential is negative with respect to the cell's resting membrane potential. Increased permeability (choices A, D, and E) would cause the cell membrane potential to depolarize. Increased Cl_ permeability (choice B) would not alter the membrane's potential.



Below figure shows an intracellularly recorded action potential from a mammalian neuron. Which of the following statements is correct concerning events taking place at the point in time indicated by the arrow?

  1. calcium conductance is higher than its normal resting level
  2. chloride conductance is higher than its normal resting level
  3. potassium conductance is higher than its normal resting level
  4. sodium conductance is higher than its normal resting level
  5. sodium-potassium ATPase activity is depressed

Answer(s): C

Explanation:

The neuron, at the arrow, is in the refractory period following the action potential. The membrane potential is more negative (hyperpolarized) than its normal resting level because potassium conductance is elevated.
The after hyperpolarization is also due to the fact that sodium channels are inactivated causing lower than normal sodium conductance (choice D). In skeletal muscle neither calcium (choice A) nor chloride (choice B) conductances should be elevated. After the action potential, intracellular sodium concentration could be slightly elevated due to the sodium influx that occurred during the action potential. This could stimulate the Na, K-ATPase (choice E).



Heparin is a rapidly acting, potent anticoagulant that has many important clinical uses. Which of the following is an action of heparin?

  1. activates prothrombin
  2. acts with antithrombin to inhibit thrombin activity
  3. decreases prothrombin time
  4. inhibits calcium action
  5. promotes vitamin K activity

Answer(s): B

Explanation:

Thrombin is a critical enzyme in the coagulation cascade. It not only can activate factors VIII and V, it also acts on fibrinogen to form fibrin. Thrombin is essential for clot formation. Antithrombin III modulates the coagulation cascade, serving to inhibit thrombin activity. Heparin acts as an anticoagulant because it accelerates the action of antithrombin III. Heparin does not stimulate vitamin K activity (choice E) and, in any case, anything that promoted vitamin K activity would increase coaguability, since vitamin K is necessary for synthesis of multiple coagulation factors. Activation of prothrombin (choice A) promotes coagulation. Though calcium is an essential cofactor for coagulation, heparin does not act via calcium inhibition (choice D). Because heparin inhibits thrombin action, it would prolong prothrombin time (choice C).






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