Diabetes Insipidus

[arc5005]

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Can someone please explain this question & answer to me:

"Diabetes insipidus (DI) is caused by an insensitivity of the kidneys to anti-diuretic hormone (ADH), or by a deficiency of that hormone. Neurogenic DI would LEAST likely exhibit?"

A. An increased plasma osmolarity.
B. A decreased urine osmolarity
C. Low plasma levels of anti-diuretic hormone
D. Decreased urine osmolarity after administration of ADH

 

[aldol16]

Anti-diuretic hormone causes retention of water because it prevents you from peeing. Insensitivity to it or deficiency of it would cause you to pee more, thereby losing more water (or solvent). This concentrates down the fluids in your body so the plasma osmolarity would increase, ruling (A) out.

Peeing more means that you're peeing out your body's solvent more, meaning that your urine is more dilute. That rules (B) out.

(C) can be ruled out because it's not entirely clear whether the hormone would be increased or decreased in the plasma. This is because if your body develops an insensitivity to the hormone, you would presumably be producing more and more of it to try to compensate but it wouldn't be working. Conversely, if your DI is from deficiency of the hormone, then your plasma levels of ADH would obviously be low because that's what defines "deficiency."

After you administer ADH, you'll be stimulated to retain water and so the urine you pee out will be more concentrated. This means (D) must be the correct answer because urine osmolarity should increase, not decrease. However, this would only be true if the DI was caused by deficiency of ADH, not if it's caused by insensitivity to the hormone.

 

[Lardy Boy]

B!


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[sazerac]

Aldol16 has the right idea here. This question hinges on knowing that neurogenic DI means that the pituitary is not producing enough ADH, versus nephrogenic DI means that the kidney itself is ignoring the (plentiful) ADH.

Since the question specifically says neurogenic DI, the answer is D. Answer C would be correct in the nephrogenic case. Answers A and B make no sense because they would appear in all cases of DI.

Is this a genuine AAMC question? Because they would usually explain in the question which scenario is neurogenic vs nephrogenic. The MCAT is supposed to be a thinking test, not a vocabulary test.

 

[arc5005]

Anti-diuretic hormone causes retention of water because it prevents you from peeing. Insensitivity to it or deficiency of it would cause you to pee more, thereby losing more water (or solvent). This concentrates down the fluids in your body so the plasma osmolarity would increase, ruling (A) out.

Peeing more means that you're peeing out your body's solvent more, meaning that your urine is more dilute. That rules (B) out.

(C) can be ruled out because it's not entirely clear whether the hormone would be increased or decreased in the plasma. This is because if your body develops an insensitivity to the hormone, you would presumably be producing more and more of it to try to compensate but it wouldn't be working. Conversely, if your DI is from deficiency of the hormone, then your plasma levels of ADH would obviously be low because that's what defines "deficiency."

After you administer ADH, you'll be stimulated to retain water and so the urine you pee out will be more concentrated. This means (D) must be the correct answer because urine osmolarity should increase, not decrease. However, this would only be true if the DI was caused by deficiency of ADH, not if it's caused by insensitivity to the hormone.

Thank you. that helps extremely.

Aldol16 has the right idea here. This question hinges on knowing that neurogenic DI means that the pituitary is not producing enough ADH, versus nephrogenic DI means that the kidney itself is ignoring the (plentiful) ADH.

Since the question specifically says neurogenic DI, the answer is D. Answer C would be correct in the nephrogenic case. Answers A and B make no sense because they would appear in all cases of DI.

Is this a genuine AAMC question? Because they would usually explain in the question which scenario is neurogenic vs nephrogenic. The MCAT is supposed to be a thinking test, not a vocabulary test.

This is a free-standing, independent question from Section V of the Biology I TBR book from this year (The Berkeley review)

 

[arc5005]

Aldol16 has the right idea here. This question hinges on knowing that neurogenic DI means that the pituitary is not producing enough ADH, versus nephrogenic DI means that the kidney itself is ignoring the (plentiful) ADH.

Since the question specifically says neurogenic DI, the answer is D. Answer C would be correct in the nephrogenic case. Answers A and B make no sense because they would appear in all cases of DI.

Is this a genuine AAMC question? Because they would usually explain in the question which scenario is neurogenic vs nephrogenic. The MCAT is supposed to be a thinking test, not a vocabulary test.

I was revisiting this question, and couldn't figure it out until I came back to SDN and searched for this, which apparently was posted by me ~ a year ago haha.