**Core Concept**
The child's presentation of sparse blond hair, developmental delay, and tremors suggests a genetic disorder affecting metabolism, particularly involving an enzyme crucial for energy production in the brain.

**Why the Correct Answer is Right**
The child's symptoms are consistent with **Tyrosinemia Type I**, caused by a deficiency of the enzyme **fumarylacetoacetate hydrolase (FAH)**. This enzyme is essential for the breakdown of the amino acid tyrosine, which accumulates to toxic levels in the body. The accumulation of tyrosine metabolites leads to liver and kidney damage, and the characteristic symptoms of developmental delay, tremors, and sparse blond hair.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because **Phenylketonuria (PKU)**, caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**, presents with mental retardation, seizures, and skin and hair abnormalities, but the hair color is typically darker.
* **Option B:** This is incorrect because **Maple Syrup Urine Disease (MSUD)**, caused by a deficiency of the enzyme **branched-chain alpha-ketoacid dehydrogenase complex (BCKD)**, presents with a characteristic sweet odor of the urine, but the hair is not typically sparse or blond.
* **Option C:** This is incorrect because **Cystinuria**, caused by a defect in the transport of cystine and other dibasic amino acids, presents with kidney stones, but the developmental delay and tremors are not typical features.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing metabolic disorders like Tyrosinemia Type I is to recognize the characteristic combination of developmental delay, tremors, and specific physical features, along with laboratory evidence of enzyme deficiency.

**Correct Answer:** A. Tyrosinemia Type I