<prompt> In acute intermittent porphyria which enzyme is deficient?ALA synthaseUroporphyrinogen I synthaseUroporphyrinogen II synthaseUroporphyrinogen III synthase  <response>: It is inherited as an autosomal dominant trait. PBG-deaminase uroporphyrinogen-I-synthase is deficient. 2. This leads to a secondary increase in the activity of ALA synthase since the end-product inhibition is not effective. The levels of ALA and PBG are elevated in blood and urine. As they are colorless compounds, urine is colorless when voided, but the color is increased on standing due to photo-oxidation of PBG to porphobilin. Hence urine samples for PBG estimation should be freshly collected and transpoed in dark bottles. Porphyrins are not excreted or elevated in blood; so there is no photosensitivity.Ref: DM Vasudevan, Page no: 246 