Question of the Week # 10

•A 55 y/o man presents with shortness of breath on exertion. Laboratory studies reveal iron deficiency anemia. Patient was started on Iron pills orally. The patient consumes a lot of red meat and was surprised to know that he was iron deficient. Fecal occult blood testing revealed a positive stool guaic. A colonoscopy and EGD were subsequently performed which were absolutely normal. A repeat Guaic was performed and was found to be negative. A further investigation for the cause of iron deficiency in this patient revealed celiac disease leading to iron malabsorption. What is the most likely cause of positive Guaic in this patient? •

A. Obscure GI Bleeding

B. Celiac disease

C. False positive from red meat

D. False positive from Iron pills

E. Colon cancer

Copy rights: Archer USMLE Reviews

10 Responses

  1. C??

  2. I would pick C as it seems reasonable. But what about B. Doesnt celiac disease also involve Fe malabsorbtion and loss?

  3. c

  4. could be both iron and red meat- confusing question

  5. Answer c meat can cause false positive FOBT iron could not produce FOBT positive. Myoglobin in red meat can case FOBT positive..

  6. C

  7. c

  8. C
    Myoglobin causes guaic +ve stool

  9. Ans. C

    Red meat can lead to false positive guaic as it may oxidize the guaic reagent ( Vitamin C on the other hand, anti-oxidizes it and causes false –ve guaic.)

    Obscure bleeding ( Ans. A) is possible but in this patient the most likely cause for iron deficiency was already established as celiac disease making intermittent obscure bleeding less likely.

    Celiac disease ( Ans. B) does not cause False +ve Guaic.

    Iron pills ( Ans. D) can cause black discoloration of the stool but not a false positive guaic.

    The fact that the colonoscopy was normal makes bleeding from colon cancer ( Ans. E)unlikely here.

  10. good q? tricky?

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