**Question:** Enzyme deficiency in Acute intermittent porphyria is

A. Alanine aminotransferase (ALT)
B. Uric acid synthase
C. Hydroxymethylglutaryl-CoA reductase (HMGCR)
D. UDP-glucuronosyl-transerase (UGT)

**Correct Answer:** C. Hydroxymethylglutaryl-CoA reductase (HMGCR)

**Core Concept:** Acute intermittent porphyria (AIP) is a rare genetic disorder characterized by deficiency of hydroxymethylglutaryl-CoA reductase (HMGCR), an enzyme involved in cholesterol synthesis. AIP is caused by mutations in the POR gene, which encodes the enzyme ferrochelatase, leading to the accumulation of uroporphyrinogen III and coproporphyrinogen III, which are precursors in heme synthesis.

**Why the Correct Answer is Right:** In AIP, the deficiency of HMGCR leads to an imbalance in heme synthesis. Specifically, the enzyme ferrochelatase is not able to incorporate ferrous iron (Fe2+) into protoporphyrin IX, a crucial step in the heme synthesis pathway. As a result, the precursors uroporphyrinogen III and coproporphyrinogen III accumulate, causing the characteristic symptoms and signs of AIP.

**Why Each Wrong Option is Incorrect:**

A. Alanine aminotransferase (ALT) is a transaminase enzyme involved in amino acid catabolism and liver function testing. It is unrelated to AIP and its enzyme deficiency.

B. Uric acid synthase is an enzyme responsible for converting xanthine to uric acid, not related to heme synthesis or AIP.

D. UDP-glucuronosyl-transerase (UGT) is involved in glucuronidation reactions in drug detoxification and bile acid conjugation, not related to heme synthesis or AIP.

**Clinical Pearl:** AIP is a rare genetic disorder that primarily affects females due to the X-linked recessive inheritance pattern. Symptoms include abdominal pain, neurological symptoms like neuropathic pain and autonomic dysfunction, and photosensitivity. Diagnosis is based on the presence of porphobilinogenuria (PBG) and delta-aminolevulinic aciduria (ALA) in urine analysis. Treatment involves avoiding triggers like alcohol, certain medications, and managing symptoms with analgesics and antiepileptics.

**Explanation of the Core Concept and each wrong option:**

**Core Concept:** Heme synthesis is a complex process involving multiple enzymes and precursors, with hydroxymethylglutaryl-CoA reductase (HMGCR) playing a critical role in the mevalonate pathway, which leads to cholesterol synthesis. In AIP, the deficiency of HMGCR results in excess accumulation of 5-aminolaevulinic acid (ALA) and porphobilinogen (PBG) in urine, leading