Question from nbme 13 NDI?? plz help

[sadaca]

Hey guys, Does anyone know how to answer this q? It seems to me its NDI so do we answer based on if ADH were present ?? Thanks for your time


5. A 22-year-old woman is admitted to the hospital because of a 10-day history of polydipsia and polyuria. She says that the urge to urinate often awakens her at night. She has been taking lithium carbonate for 2 years for bipolar disorder; her dosage was increased 6 months ago because of recurrent severe manic episodes. Her vital signs are within normal limits. Physical examination shows no abnormalities. Over the next 24 hours, urine excretion totals 6.5 L. Laboratory studies at this time show a serum sodium concentration of 148 mEq/L, serum osmolality of 315 mOsmol/kg, and urine osmolality of 75 mOsmol/kg. After administration of desmopressin, urine output and osmolality do not change. Which of the following findings in the nephron best describes the tubular osmolality, compared with serum, in this patient?

Proximal Tubule Juxtaglomerular Apparatus Medullary Collecting Duct

A) Hypertonic hypertonic hypertonic

B) Hypertonic hypertonic hypotonic

C) Hypertonic hypotonic hypotonic

D) Isotonic isotonic isotonic

E) Isotonic hypotonic hypertonic

F) Isotonic hypotonic hypotonic

G) Hypotonic hypertonic hypertonic

H) Hypotonic hypotonic hypertonic

I) Hypotonic hypotonic hypotonic

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[DrShazam]

You're right, it is nephrogenic DI. The question is asking you what the values are in the absence of ADH action. You know the fluid is hypotonic in the medullary collecting duct since the kidneys are not responding to ADH and you're not absorbing any water. That gets you down to B, C, F. Now you also know the fluid in the proximal tubule is isotonic since ADH doesn't act on this, and the fluid in the PT is normally isotonic. So the correct answer is F.

 

[sadaca]

Hey Thanks alot @DrShazam . My question is - its NDI, where ADH is normal but it is just not acting. So how did you figure they want values for when ADH is absent??
Appreicate it,

 

[DrShazam]

As you mentioned, for nephrogenic DI, ADH is present but it's not acting. That's like ADH not being there at all. That's how I knew what they were asking for-- they're asking what the values look like in nephrogenic DI, which would not be any different than had the question presented central DI. Only difference between the two is whether you can correct it with desmopressin. Hope this helps.

 

[sadaca]

Thank you @DrShazam That was very helpful. One last question here- The PT is always isotonic with/without ADH, correct?

 

[DrShazam]

Sadaca, right again The PT is always isotonic, doesn't matter whether ADH is present or not.

 

[sadaca]

ah gotta trust myself more! Thanks @DrShazam , you rock!

 

[britesky89]

@ Dr. Shazam, how do you explain the hypotonicity of the JGA in nephrogenic DI?

 

[Treebeard]

@ Dr. Shazam, how do you explain the hypotonicity of the JGA in nephrogenic DI?

Sorry to necrobump, but I've been trying to figure this out as well. I got the question correct because the PT and MCD parts definitely make sense to me and luckily there was only 1 reasonable option based on those. However, I can't for the life of me figure out how the JGA is becoming hypotonic?

 

[Jesus1]

The JGA is somewhat a part of the distal convoluted tubule and the Na/K/2Cl transporter at the thick ascending limb of the loop of henle has already reabsorbed some salt. Hope this helps.