All of the following types of porphyria are autosomal dominant except:
**Core Concept**
Porphyrias are a group of disorders resulting from defects in the biosynthesis of heme, leading to the accumulation of porphyrins or their precursors. Autosomal dominant inheritance means that a single copy of the mutated gene is sufficient to cause the condition, and each offspring has a 50% chance of inheriting the mutated gene.
**Why the Correct Answer is Right**
Acute intermittent porphyria (AIP) is the most common form of porphyria and is inherited in an autosomal dominant manner. The defect in AIP lies in the _HMBS_ gene, which encodes the enzyme hydroxymethylbilane synthase (porphobilinogen deaminase). This enzyme is crucial for the conversion of porphobilinogen to hydroxymethylbilane in the heme biosynthesis pathway. Mutations in the _HMBS_ gene lead to the accumulation of porphobilinogen and other porphyrin precursors, resulting in the characteristic symptoms of AIP.
**Why Each Wrong Option is Incorrect**
**Option B:** Porphyria cutanea tarda (PCT) is indeed an autosomal dominant condition, although the expression of the disease can be influenced by environmental factors such as alcohol consumption and iron overload. However, the question is looking for a type of porphyria that is not autosomal dominant.
**Option C:** Congenital erythropoietic porphyria (CEP) is another type of porphyria that is inherited in an autosomal recessive manner, not autosomal dominant. This condition is caused by mutations in the _UROD_ gene, which encodes the enzyme uroporphyrinogen decarboxylase.
**Option D:** (Blank) This option is not a type of porphyria, so it cannot be considered as a correct answer.
**Clinical Pearl / High-Yield Fact**
Porphyrias can be classified into two main categories: acute (e.g., AIP) and cutaneous (e.g., PCT). Acute porphyrias are characterized by symptoms such as abdominal pain, neurological symptoms, and psychiatric disturbances, whereas cutaneous porphyrias are characterized by blistering skin lesions and photosensitivity.
**Correct Answer:** C. Congenital erythropoietic porphyria (CEP) is the type of porphyria that is autosomal recessive, not autosomal dominant.