**Core Concept**
Autosomal dominant disorders are characterized by a mutation in one of the two copies of a gene, leading to the expression of the condition. This type of inheritance pattern is often seen in conditions where a single copy of the mutated gene is sufficient to cause the disease. Autosomal dominant disorders are typically inherited in a vertical pattern, affecting multiple generations.

**Why the Correct Answer is Right**
Phenylketonuria (PKU) is an autosomal recessive disorder, not autosomal dominant. It is caused by a mutation in the PAH gene, which codes for the enzyme phenylalanine hydroxylase. This enzyme is essential for the breakdown of the amino acid phenylalanine. In PKU, the mutated enzyme is either non-functional or has reduced activity, leading to the accumulation of phenylalanine in the body. This condition requires strict dietary management to prevent brain damage and other complications. The other options, Familial hypercholesterolemia, Hereditary spherocytosis, and Acute intermittent porphyria, are all autosomal dominant disorders.

**Option A:** Familial hypercholesterolemia is an autosomal dominant disorder caused by mutations in the LDLR gene, which codes for the low-density lipoprotein receptor. This receptor plays a crucial role in removing LDL cholesterol from the bloodstream.

**Option B:** Hereditary spherocytosis is an autosomal dominant disorder caused by mutations in genes encoding proteins of the red blood cell membrane, such as spectrin, ankyrin, and band 3.

**Option C:** Acute intermittent porphyria is an autosomal dominant disorder caused by mutations in the HMBS gene, which codes for the enzyme hydroxymethylbilane synthase. This enzyme is involved in the production of heme, a crucial component of hemoglobin.

**Clinical Pearl / High-Yield Fact**
Autosomal dominant disorders often have a higher penetrance than autosomal recessive disorders, meaning that a larger proportion of individuals with the mutated gene will express the condition.

**✓ Correct Answer: D. Phenylketonuria is an autosomal recessive disorder, not autosomal dominant.**