A 4 year old girl presented to hospital with failure to thrive. Her peripheral blood smear showed megaloblastic anemia. Vitamin B12 & folate were given but anemia did not improve. Enzyme assay from the cultured PBMC showed deficiency of OP (Orotate Phospho Ribosyl Transferase). What could be the probable diagnosis ?

**Core Concept**
Megaloblastic anemia is characterized by the production of abnormally large red blood cells that are unable to function properly, often due to deficiencies in either vitamin B12 or folate. However, the girl's response to vitamin B12 and folate supplementation suggests an alternative cause of megaloblastic anemia. The enzyme assay showing a deficiency of orotate phosphoribosyltransferase (OPRT) is crucial in identifying the underlying pathophysiology.

**Why the Correct Answer is Right**
The enzyme orotate phosphoribosyltransferase (OPRT) is involved in the pyrimidine synthesis pathway, specifically in the conversion of orotate to orotidine-5'-phosphate. A deficiency in OPRT leads to a block in pyrimidine synthesis, which can cause megaloblastic anemia due to impaired DNA synthesis and cell division. The girl's failure to respond to vitamin B12 and folate supplementation is consistent with this diagnosis, as the underlying issue is not related to these vitamins but rather a defect in pyrimidine synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the enzyme deficiency or the pyrimidine synthesis pathway.
**Option B:** This option is incorrect because it does not account for the girl's failure to respond to vitamin B12 and folate supplementation.
**Option C:** This option is incorrect because it does not provide a plausible explanation for the OPRT deficiency.

**Clinical Pearl / High-Yield Fact**
The diagnosis of pyrimidine synthesis defects, including OPRT deficiency, is often challenging due to their rarity and nonspecific clinical presentation. However, a high index of suspicion and a detailed metabolic workup, including enzyme assays, can lead to the correct diagnosis and appropriate management.

**Correct Answer:** D.

✓ Correct Answer: C. Orotic aciduria