Malabsorption of B12 vs Iron

[Enzymes]

I just don't get this question that I recently encountered on UWorld (ID: 786). Basically it was a B12 deficiency due to malabsorption (since this woman had normal diet). The signs and symptoms of fatigue were progressive, but I am not sure why that makes B12 more likely than Iron. There were no indications that she had a megaloblastic anemia or neuropathy, so I ended up choosing Iron deficiency. I know this isn't exactly the best question / minutia, but can someone explain how they approached this problem / ruled out iron deficiency?

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[Ismet]

The question asked which of the following supplements requires parenteral administration, not what deficiency she had.

B12 in her case required parenteral administration due to malabsorption. There's no point in giving her oral B12 if she can't absorb it. Iron is given orally unless the patient can't tolerate it or if they have such a huge deficit that it needs to be administered immediately. All of this is stated in the explanation of the answer.

 

[bulldogmed]

w/o the specifics of the question, it's hard to say, but iron, folate, and B12 all absorb at different points in the SI. Iron in the duodenum, folate in the jejunum and B12 in the ileum.

 

[Enzymes]

The question asked which of the following supplements requires parenteral administration, not what deficiency she had.

B12 in her case required parenteral administration due to malabsorption. There's no point in giving her oral B12 if she can't absorb it. Iron is given orally unless the patient can't tolerate it or if they have such a huge deficit that it needs to be administered immediately. All of this is stated in the explanation of the answer.

Thanks for the response. The thing is, to me, the deficiency is clearly due to malabsorption, so I was in fact thinking more along the lines of which deficiency the patient had (not that I was convinced by either B12 or Iron). In a malabsorptive process, obviously you need to give IV injections of whatever is not being absorbed. Why would you give oral Fe if you can't absorb it in the first place? Am I missing something here?

 

[Ismet]

Unfortunately it's a matter of which is the "best" answer, even if multiple answers are plausible. Yes, if there is iron malabsorption, you'd give parenteral iron. Her symptoms weren't exactly specific in either direction, but if she had an iron malabsorption, I think her symptoms would be much more severe and rapid, rather than a gradual progression over several years. The most likely etiology of the anemia in the vignette is B12 malabsorption. Rather poor question IMO.

 

[Enzymes]

Unfortunately it's a matter of which is the "best" answer, even if multiple answers are plausible. Yes, if there is iron malabsorption, you'd give parenteral iron. Her symptoms weren't exactly specific in either direction, but if she had an iron malabsorption, I think her symptoms would be much more severe and rapid, rather than a gradual progression over several years. The most likely etiology of the anemia in the vignette is B12 malabsorption. Rather poor question IMO.

Thanks a lot. I completely agree. Usually when I see "progressive" in a stem, I am thinking B12...but the fact that she had no specific symptoms really irritated me. Nice working with ya.