**Question:** A child who had derangement of milestones and was hyperactive later on developed self-mutilating behavior and hair pulling behavior. The child died a few years later. Which of the following enzyme is most likely deficient in the child?

A. GAD (Glutamic Acid Decarboxylase)
B. MAO (Monoamine Oxidase)
C. DOPA Decarboxylase
D. DDC (Decarboxylase)

**Correct Answer:** C. DOPA Decarboxylase

**Core Concept:**
The correct answer is **C** - DOPA Decarboxylase (DDC). DDC is an enzyme responsible for the conversion of L-DOPA (L-3,4-dihydroxyphenylalanine) into dopamine, which is a precursor to norepinephrine and epinephrine as well. This enzyme plays a crucial role in the synthesis of neurotransmitters and their derivatives, which are essential for proper brain function and development.

**Why the Correct Answer is Right:**
In this case, the child shows a combination of symptoms, including hyperactivity, self-mutilating behavior, and hair pulling, which are indicative of a deficiency in the production of neurotransmitters. Among the given options, DOPA Decarboxylase is the most relevant enzyme involved in the synthesis of dopamine, norepinephrine, and epinephrine.

**Why Each Wrong Option is Incorrect:**

A. GAD (Glutamic Acid Decarboxylase): This enzyme is involved in the synthesis of GABA (Gamma-Aminobutyric Acid) and does not contribute to the synthesis of neurotransmitters associated with the child's symptoms.

B. MAO (Monoamine Oxidase): This enzyme is involved in the degradation of various neurotransmitters, not their synthesis, which is the focus of the question.

D. DDC (Decarboxylase): Although DDC is involved in the synthesis of dopamine, it is not the enzyme that is directly responsible for the synthesis of norepinephrine and epinephrine.

**Clinical Pearl:**
The child's symptoms of hyperactivity, self-mutilating behavior, and hair pulling are indicative of a deficiency in the synthesis of dopamine, norepinephrine, and epinephrine. This deficiency can be caused by a genetic defect in the enzyme responsible for their synthesis, such as DDC (Decarboxylase) or MAO (Monoamine Oxidase). These deficiencies may lead to the observed symptoms due to the reduced availability of these neurotransmitters in the child's brain, causing dysfunction in normal brain development and functioning.