Downey cell is seen in –
**Downey Cell**
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**Core Concept**
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Downey cells are a type of abnormal red blood cell seen in the peripheral blood smear of patients with pernicious anemia, a condition caused by a deficiency of vitamin B12. These cells are characterized by their large size, oval or sausage-shaped morphology, and the presence of a central pallor.
**Why the Correct Answer is Right**
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Downey cells are a specific type of megaloblastic anemia, which is caused by impaired DNA synthesis due to a lack of vitamin B12. The deficiency leads to the inhibition of the enzyme methylmalonyl-CoA mutase, which is necessary for the conversion of methylmalonyl-CoA to succinyl-CoA. This, in turn, results in the accumulation of megaloblasts, which are large, immature red blood cells that eventually become Downey cells.
**Why Each Wrong Option is Incorrect**
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**Option A:** This option is incorrect because Downey cells are not typically seen in iron-deficiency anemia, which is characterized by microcytic, hypochromic red blood cells.
**Option B:** This option is incorrect because sideroblasts are a type of abnormal red blood cell seen in sideroblastic anemia, characterized by the presence of iron granules in the mitochondria of red blood cell precursors.
**Option C:** This option is incorrect because schistocytes are fragmented red blood cells seen in conditions such as microangiopathic hemolytic anemia.
**Clinical Pearl / High-Yield Fact**
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Downey cells are a classic example of a megaloblastic anemia, which can be caused by a deficiency of either vitamin B12 or folate.
**Correct Answer:** D. Pernicious anemia