**Core Concept**
Phenylketonuria (PKU) is an inherited metabolic disorder caused by deficiency of the enzyme phenylalanine hydroxylase, leading to accumulation of phenylalanine and toxic metabolites. This results in neurodevelopmental impairment, especially when untreated, and is often associated with fair skin, hypopigmented hair, and eczema—key features in early diagnosis.

**Why the Correct Answer is Right**
In PKU, the lack of phenylalanine hydroxylase prevents conversion of phenylalanine to tyrosine. This leads to high blood phenylalanine levels, which are neurotoxic and cause intellectual disability, irritability, and developmental delays. The hypopigmentation (fair skin, blue eyes, hypopigmented hair) and eczema are hallmark signs, especially in infants. Early detection via newborn screening and strict low-phenylalanine diet are critical to prevent irreversible neurological damage.

**Why Each Wrong Option is Incorrect**
Option A: Albinism involves genetic defects in melanin production, leading to severe hypopigmentation and photophobia, but not eczema or irritability. It does not present with elevated phenylalanine or neurodevelopmental delay from metabolic toxicity.
Option C: Alkaptonuria causes darkening of urine on oxidation and osteoarthritis, not eczema or developmental delay. Hypopigmentation is not a feature.
Option D: Cystinosis presents with corneal crystals, chronic eye disease, and early-onset joint pain, not skin pigmentation or developmental delay.

**Clinical Pearl / High-Yield Fact**
In infants with developmental delay, fair skin, blue eyes, and eczema, **PKU must be suspected immediately**—especially if there is a history of irritability or poor feeding. Newborn screening for phenylalanine levels is mandatory and should be followed by dietary intervention within days of birth.

✓ Correct Answer: B. Phenylketonuria