Four vitamins and enzymes, one smear. If you know which step broke, the labs, the symptoms and the treatment follow, and you stop memorizing them as separate facts.
In megaloblastic anemia, thymidylate synthase turns dUMP into dTMP using methylene-THF. RNA and protein synthesis do not need thymine, so the cytoplasm grows while the nucleus stalls.
Four doors lead to the same stall: B12 deficiency with the methyl-folate trap, folate deficiency, a B12 or folate metabolism defect, or orotic aciduria with no UMP to start from.
B12 gives high homocysteine, high MMA and neurologic signs. Folate gives high homocysteine with normal MMA. Orotic aciduria gives urine orotic acid with both normal.
The trap is treating a B12 patient with folic acid. The blood improves and the spinal cord keeps degenerating. The same logic also shows up as a methotrexate or 5-FU question.