Pathology Cheat Sheet
Dna Synthesis Leaves a Smear
Megaloblastic anaemia for NEET-PG: B12 and folate, intrinsic factor, pernicious anaemia, hypersegmented neutrophils, and subacute combined degeneration.
MedNext Academy | 3 min read
Dna Synthesis Leaves a Smear
Megaloblastic anaemia for NEET-PG: B12 and folate, intrinsic factor, pernicious anaemia, hypersegmented neutrophils, and subacute combined degeneration.
Vitamin B12 and folate metabolism, the distinction of megaloblastic from non-megaloblastic macrocytosis, and their blood and marrow findings.
High-yield lines
- Megaloblastic anaemia results from impaired DNA synthesis due to vitamin B12 or folate deficiency.
- Vitamin B12 is found only in animal-source foods, so strict vegans are at risk of deficiency.
- Dietary B12 deficiency is the most common cause of megaloblastic anaemia in India due to widespread vegetarian diets.
- Intrinsic factor from gastric parietal cells is required for B12 absorption in the terminal ileum.
- Pernicious anaemia is autoimmune destruction of parietal cells causing intrinsic factor deficiency.
- The peripheral smear in megaloblastic anaemia shows macro-ovalocytes and hypersegmented neutrophils with six or more nuclear lobes.
- The bone marrow in megaloblastic anaemia shows nuclear-cytoplasmic asynchrony with large immature nuclei and mature cytoplasm.
- B12 deficiency causes subacute combined degeneration of the spinal cord affecting the dorsal and lateral columns.
- Folate deficiency causes megaloblastic anaemia without the neurological features of B12 deficiency.
- Elevated methylmalonic acid and homocysteine indicate B12 deficiency, while folate deficiency raises only homocysteine.
- Non-megaloblastic macrocytosis occurs in liver disease, alcoholism, hypothyroidism, and reticulocytosis.
- Megaloblastic anaemia produces ineffective erythropoiesis with intramedullary haemolysis raising bilirubin and LDH.
Mapped competency codes
- PA15.1
- PA15.2
- PA15.3
- PA15.4
Continue into the full chapter
This summary maps to PA15-macrocytic-anemia.
Frequently Asked Questions
What is the smear hallmark of megaloblastic anaemia?
Macro-ovalocytes together with hypersegmented neutrophils having six or more nuclear lobes, reflecting impaired DNA synthesis.
How does B12 deficiency differ from folate deficiency clinically?
Both cause megaloblastic anaemia, but B12 deficiency also causes subacute combined degeneration of the spinal cord, which folate deficiency does not.
What is pernicious anaemia?
An autoimmune disease in which antibodies destroy gastric parietal cells and intrinsic factor, causing failure of vitamin B12 absorption in the terminal ileum.
How do laboratory metabolites separate B12 from folate deficiency?
B12 deficiency raises both methylmalonic acid and homocysteine, whereas folate deficiency raises only homocysteine.
Continue reading
Cheat SheetsAll Pathology cheat sheets
Browse the complete authored set for this subject.
Open the complete revision pathway
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans

