A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 45-year-old man undergoes an esophagogastroduodenoscopy for his recurrent episodes of epigastric pain. He also lost a significant amount of weight in the last 6 months. He says that he has been taking a number of dietary supplements "to cope". His past medical history is insignificant, and a physical examination is within normal limits. The endoscopy shows a bleeding ulcer in the proximal duodenum. Lab tests reveal a serum iron level of 130 μg/dL. However, his stool guaiac test is negative for occult blood. Over-ingestion of which of the following substances is the most likely cause for this patient’s lab findings?
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A
FolateIncorrect. Folate excess does not raise serum iron levels or interfere with guaiac testing.
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B
ThiamineIncorrect. Thiamine excess is essentially nontoxic and produces no relevant lab abnormalities.
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C
Ascorbic acidCorrect. Ascorbic acid both enhances duodenal iron absorption (raising serum iron) and causes a false-negative stool guaiac by interfering with the peroxidase chromogen reaction.
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D
TocopherolIncorrect. Tocopherol (vitamin E) excess causes bleeding diathesis via inhibition of vitamin K-dependent clotting factors, not increased iron or false-negative guaiac.
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E
NiacinIncorrect. Niacin excess causes flushing, pruritus, and hepatotoxicity, but does not increase duodenal iron absorption or produce false-negative guaiac results.
↑ Tap an answer to reveal the reasoning
Answer: C. A patient with epigastric pain, weight loss, and an actively bleeding duodenal ulcer should have iron-deficiency anemia from chronic blood loss — yet his serum iron is elevated at 130 µg/dL and the stool guaiac is falsely negative. The unifying explanation is over-ingestion of vitamin C (ascorbic acid) from his dietary supplements.
Ascorbic acid has two relevant effects here. First, it reduces dietary Fe3+ to the absorbable Fe2+ form in the duodenum, dramatically increasing iron absorption and producing an elevated serum iron despite ongoing GI blood loss. Second, vitamin C is a known cause of false-negative guaiac (occult blood) testing — it interferes with the peroxidase reaction used to detect heme by reducing the indicator dye, masking otherwise detectable bleeding.
This combination — supplement-using patient, falsely normal/elevated iron in the face of a bleeding ulcer, and a false-negative stool guaiac — is essentially pathognomonic for excess ascorbic acid intake. Clinicians should specifically ask patients to avoid vitamin C for several days before guaiac testing to prevent this artifact.
**Why each option:**
**A.** Folate excess does not raise serum iron levels or interfere with guaiac testing.
**B.** Thiamine excess is essentially nontoxic and produces no relevant lab abnormalities.
**C.** Correct. Ascorbic acid both enhances duodenal iron absorption (raising serum iron) and causes a false-negative stool guaiac by interfering with the peroxidase chromogen reaction.
**D.** Tocopherol (vitamin E) excess causes bleeding diathesis via inhibition of vitamin K-dependent clotting factors, not increased iron or false-negative guaiac.