## Correct Answer Analysis **Key Point:** The primary neurochemical abnormality in ADHD is DECREASED (not increased) dopaminergic and noradrenergic transmission in the prefrontal cortex. This is the fundamental rationale for stimulant therapy. ## Neurochemical Model of ADHD ### The Catecholamine Hypothesis **High-Yield:** ADHD is characterized by **hypofunction** of dopamine and noradrenaline systems in the prefrontal cortex and anterior cingulate. This leads to: - Impaired executive function (working memory, inhibition, planning) - Reduced sustained attention - Difficulty with impulse control **Mechanism of Stimulant Action:** Stimulants (methylphenidate, amphetamine) INCREASE dopamine and noradrenaline availability by blocking reuptake, thereby correcting the underlying deficit. If transmission were already elevated, stimulants would worsen symptoms — but they improve them, confirming the hypofunction model. ## Why Each Other Option IS Correct ### Option 0: Structural Brain Abnormalities **Clinical Pearl:** Meta-analyses of neuroimaging in ADHD consistently show reduced grey matter volume in: - Anterior cingulate cortex (involved in attention and error monitoring) - Dorsolateral prefrontal cortex (executive function, working memory) - Posterior cingulate and precuneus These findings are well-replicated and correct. ### Option 1: Default Mode Network Dysfunction **Key Point:** The default mode network (DMN) — which includes the anterior insula, medial prefrontal cortex, and posterior cingulate — shows abnormal activity patterns in ADHD. Specifically: - Hyperactivity of the anterior insula during rest ("mind-wandering") - Delayed maturation of DMN connectivity - Poor suppression of DMN during task performance (contributing to distractibility) This is a recognized neurobiological marker and is correct. ### Option 3: Heritability and Genetics **High-Yield:** ADHD has a heritability of approximately 70–80%, making it one of the most heritable psychiatric disorders (comparable to autism and schizophrenia). Twin and family studies consistently support this. Multiple genes (polygenic inheritance) are implicated, including those affecting dopamine and noradrenaline signalling. This is correct. [cite:Sadock, Sadock & Ruiz 11e Ch 35]
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