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Free STEP1 Exam Braindumps (page: 61)

Page 60 of 213
PDF with 845 Questions

Which of the following is a true statement regarding renal clearance of a substance?

  1. is measured in milligram per minute
  2. glucose is greater than that of insulin in the normal kidney
  3. insulin is zero
  4. PAH remains constant regardless of the plasma level
  5. potassium clearance will increase after an injection of aldosterone

Answer(s): E

Explanation:

Choice E is correct since the clearance of potassium is normally regulated by the action of aldosterone on the distal tubule exchange of sodium reabsorption with potassium secretion. Choice Ais incorrect because renal clearance represents the volume of plasma completely cleared of a substance per unit time, hence the units for renal clearance are milliliter per minute, and not milligram per minute. Choice B is incorrect because a normal kidney reabsorbs 100% of the filtered glucose, so glucose clearance is zero. By contrast, a significant amount of inulin is usually cleared by the kidney. Choice C is incorrect because although inulin is not secreted by the tubule as is PAH, inulin is filtered but not reabsorbed and thus a normal kidney will clear approximately 20% of the inulin from the plasma. Choice D is incorrect because PAH is actively secreted by the proximal tubule. Above a certain plasma level, PAH secretion no longer can increase and thus clearance (calculated as the ratio of urine to plasma concentration multiplied by urine flow) will decline as plasma PAH concentration increases faster than urine PAH concentration.



As part of a neurological examination, a patient is placed in a chair that rotates to the right under the control of a computer so that the patient's head and body move in unison with the chair. How would a healthy person react immediately upon getting off the chair that turned for 1 minute?

  1. behave the same as before the rotation
  2. display post-rotational eye nystagmus to the right
  3. experience a vertigo that would be diminished if he immediately got back in the rotating chair
  4. experience "past pointing" to the left when asked to reach a target
  5. fall to the left when asked to walk on a straight line

Answer(s): C

Explanation:

For about 2030 seconds after the rotation stops, the endolymph of the vestibular system continues to rotate due to its inertia. The brain still receives the input of rotation, while the input from the person's eyes and mechanoreceptors signal that the motion stopped. This conflicting information causes dizziness which would disappear if the person continue rotating. The eyes of a healthy person would be "flicking" in the direction opposite to the direction of rotation, so that nystagmus would be to the left, not to the right (choice B). If the person attempts to reach out and touch a target during the postrotatory nystagmus, the movement will miss the target in the direction of the slow phase of nystagmus, to the right in this case, not to the left (choice D). An attempt to walk during the period of nystagmus and vertigo induced by ceasing rotation often leads to a fall, in this case to the right, not to the left (choice E). The compensatory movements to the right are due to the fact that the brain interprets the continuing rotation of the endolymph in relation to the now unmoving outer walls of the vestibular organs as if a rotation to the left has begun. This incorrect input signal is processed by the brain to influence eyes, trunk, and limb muscles to exert compensatory actions.
The responses are entirely normal and their absence could indicate dysfunction of the peripheral or central vestibular system (choice A).



One of your diabetic patients has a blood glucose level of 200 mg/dL. Surprisingly, a dipstick test is negative for urinary glucose. How could this finding be explained?

  1. dipstick tests are more sensitive for reducing sugars other than glucose
  2. the patient has defective tubular glucose transporters
  3. the patient has diabetes insipidus
  4. the patient has significantly reduced GFR
  5. the patient is in a state of antidiuresis

Answer(s): D

Explanation:

Glucose excretion by the kidneys depends on the glomerular filtration and tubular reabsorption rates.
Glucose first appears in the urine, when the capacity of the glucose transporters in the proximal tubuli cells is exceeded. This usually occurs at plasma glucose levels higher than 180 mg/dL. Patients with longstanding diabetes mellitus often have decreased renal function and reduced GFR. Under these circumstances the threshold (i.e., plasma level) for excretion of glucose is higher than in a healthy person.
Urine dipsticks nowadays are both sensitive and specific for glucose, detecting as little as 100 mg/dL (choice A). Earlier dipstick tests were sensitive but not specific (they detected other reducing sugars in addition to glucose). Adefect in glucose transporters (choice B) results in glucosuria even at normal plasma glucose concentration. Patients with diabetes insipidus (choice C) have a large urine output due to absence of ADH or defective renal ADH receptors, but should not have a plasma glucose level of 200 mg/dL.
Antidiuresis (choice E) increases the concentration of solutes in the urine and increases the sensitivity to detect urine glucose. Reabsorption of filtered glucose occurs in the proximal tubule by active transport.



Exhibit:

Please refer to the exhibit:
Which of the following is a feature of the principal cell in the cortical collecting duct?

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

Answer(s): D

Explanation:

The principal cell is the major sodiumreabsorbing and potassium-secreting cell in the collecting duct. It functions via specific sodium and potassium channels that increase luminal cell membrane permeabilty to these ions and a sodium/potassium ATPase in the basolateral cell membrane. Choice A is incorrect because the specific movement of sodium through the luminal membrane via specific sodium channels creates an excess of negative charges in the tubular lumen. Choice B is incorrect because sodium is reabsorbed while potassium is predominantly secreted by this cell. Choice C is incorrect because aldosterone regulates sodium and potassium transport, not water transport. ADHdoes increase water transport via aquaporins. Choice E is incorrect because a sodium/potassium/2 chloride transporter is not located in this cell type, but is located in the thick ascending loop of Henle cells instead.






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