## Why "Fair skin, blue eyes, and musty/mousy body odor due to phenylacetate accumulation" is right Phenylalanine hydroxylase (PAH; marked **A**) catalyzes the conversion of phenylalanine to tyrosine. Deficiency of PAH causes phenylketonuria (PKU), an autosomal recessive disorder. Accumulated phenylalanine is transaminated to phenylpyruvate and decarboxylated to phenylacetate, which is excreted in urine and produces the characteristic musty or mousy odor. Because tyrosine production is reduced, melanin synthesis is impaired, resulting in fair skin, blonde hair, and blue eyes. This phenotype is pathognomonic for untreated PKU and is explicitly documented in Harper 32e Ch 29 and Nelson 21e Ch 103. ## Why each distractor is wrong - **Dark skin pigmentation with oculocutaneous albinism and photophobia**: This describes oculocutaneous albinism caused by deficiency of tyrosinase (marked **B**), not PAH. Tyrosinase catalyzes melanin synthesis from tyrosine; its deficiency results in complete absence of pigment, not the fair skin of PKU. - **Dark urine on standing and ochronosis of connective tissues**: This is the classic presentation of alkaptonuria, caused by deficiency of homogentisate oxidase (marked **C**). Homogentisic acid accumulates, oxidizes, and polymerizes to form a dark pigment that deposits in connective tissues. - **Hepatomegaly, renal tubular dysfunction, and cabbage-like odor**: This describes tyrosinemia type I, caused by deficiency of fumarylacetoacetate hydrolase (marked **D**). The cabbage-like odor is from methionine accumulation, not phenylacetate. **High-Yield:** PKU = fair skin + blue eyes + musty odor + intellectual disability if untreated; newborn screening (Guthrie test) is mandatory in India; lifelong phenylalanine-restricted diet is the cornerstone of management. [cite: Harper 32e Ch 29; Nelson 21e Ch 103]
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