**Core Concept**
Mousy odour urine is associated with a specific metabolic disorder that affects the metabolism of certain amino acids, leading to the accumulation of a compound responsible for the characteristic odour.

**Why the Correct Answer is Right**
The correct answer is related to Phenylketonuria (PKU), a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH). This enzyme is crucial for the metabolism of the amino acid phenylalanine (Phe) to tyrosine. In PKU, the accumulation of phenylalanine and its metabolites, such as phenylacetate, leads to the characteristic mousy odour in urine, as well as intellectual disability and other systemic manifestations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the mousy odour urine.
**Option B:** While some metabolic disorders can cause unusual urine odours, they are not typically associated with a mousy smell.
**Option C:** This option may be a distractor, but it is not the correct answer.

**Clinical Pearl / High-Yield Fact**
PKU is an autosomal recessive disorder, and early screening and treatment can prevent intellectual disability and other complications. The classic clinical triad of PKU includes mousy odour urine, intellectual disability, and eczema.

**Correct Answer: D. Phenylketonuria (PKU).**