A 35-year-old man with severe megaloblastic anemia due to vitamin B12 deficiency has serum LDH of 2400 U/L. Which isoenzyme pattern accounts for this marked elevation?
- A Marked elevation of LDH1 and LDH2 from ineffective erythropoiesis and intramedullary cell destruction ✓
- B Isolated LDH5 elevation from hepatic congestion
- C Equal elevation of all five fractions indicating diffuse tissue necrosis
- D LDH4 and LDH5 elevation from renal infarction
Correct answer: A. Marked elevation of LDH1 and LDH2 from ineffective erythropoiesis and intramedullary cell destruction
Explanation
In megaloblastic anemia, defective DNA synthesis causes massive intramedullary destruction of megaloblastic precursors, releasing large amounts of the LDH1 and LDH2 isoenzymes characteristic of erythroid cells. Serum LDH can exceed ten times the upper limit, far higher than in hemolytic anemia, and this magnitude is a classic diagnostic clue. LDH5 reflects liver and skeletal muscle, neither of which is involved here.
Reference: Harper's Illustrated Biochemistry, 31st ed.
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