A mother brings her 3-year-old son to the doctor because she is worried that he might be harming himself by constantly banging his head on the wall. He has been exhibiting this behavior for a few months. She is also worried because he has started to speak less than he used to and does not respond when his name is called. He seems aloof during playtime with other children and seems to have lost interest in most of his toys. What is the most likely diagnosis?

Attention deficit hyperactivity disorder

A 7-year-old boy is brought to the physician because of repetitive, involuntary blinking, shrugging, and grunting for the past year. His mother states that his symptoms improve when he is physically active, while tiredness, boredom, and stress aggravate them. He has felt increasingly embarrassed by his symptoms in school, and his grades have been dropping from average levels. He has met all his developmental milestones. Vital signs are within normal limits. Mental status examination shows intact higher mental functioning and thought processes. Excessive blinking, grunting, and jerking of the shoulders and neck occur while at rest. The remainder of the examination shows no abnormalities. This patient's condition is most likely associated with which of the following findings?

Excessive impulsivity and inattention

Feelings of persistent sadness and loss of interest

Defiant and hostile behavior toward teachers and parents

Recurrent episodes of intense fear

A 14-year-old boy is brought in to the clinic by his parents for weird behavior for the past 4 months. The father reports that since the passing of his son's pet rabbit about 5 months ago, his son has been counting during meals. It could take up to 2 hours for him to finish a meal as he would cut up all his food and arrange it in a certain way. After asking the parents to leave the room, you inquire about the reason for these behaviors. He believes that another family member is going to die a “terrible death” if he doesn’t eat his meals in multiples of 5. He understands that this is unreasonable but just can’t bring himself to stop. Which of the following abnormality is this patient's condition most likely associated with?

Atrophy of the frontotemporal lobes

Increased activity of the caudate

A 7-year-old boy is brought in to clinic by his parents with a chief concern of poor performance in school. The parents were told by the teacher that the student often does not turn in assignments, and when he does they are partially complete. The child also often shouts out answers to questions and has trouble participating in class sports as he does not follow the rules. The parents of this child also note similar behaviors at home and have trouble getting their child to focus on any task such as reading. The child is even unable to watch full episodes of his favorite television show without getting distracted by other activities. The child begins a trial of behavioral therapy that fails. The physician then tries pharmacological therapy. Which of the following is most likely the mechanism of action of an appropriate treatment for this child's condition?

Decreases synaptic reuptake of norepinephrine and dopamine

Increases the frequency of GABAa channel opening

Increases the duration of GABAa channel opening

A 9-year-old boy is brought to a pediatric psychologist by his mother because of poor academic performance. The patient’s mother mentions that his academic performance was excellent in kindergarten and first grade, but his second and third-grade teachers complain that he is extremely talkative, does not complete schoolwork, and frequently makes careless mistakes. They also complain that he frequently looks at other students or outside the window during the class and is often lost during the lessons. At home, he is very talkative and disorganized. When the pediatrician asks the boy his name, he replies promptly. He was born at full term by spontaneous vaginal delivery. He is up-to-date on all vaccinations and has met all developmental milestones on time. A recent IQ test scored him at 95. His physical examination is completely normal. When he is asked to read from an age-appropriate children’s book, he reads it fluently and correctly. Which of the following is the most likely diagnosis in this patient?

Attention-deficit/hyperactivity disorder

A 19-year-old man presents to the emergency room after a suicidal gesture following a fight with his new girlfriend. He tearfully tells you that she is “definitely the one," unlike his numerous previous girlfriends, who were "all mean and selfish” and with whom he frequently fought. During this fight, his current girlfriend suggested that they spend time apart, so he opened a window and threatened to jump unless she promised to never leave him. You gather that his other relationships have ended in similar ways. He endorses impulsive behaviors and describes his moods as “intense” and rapidly changing in response to people around him. He often feels “depressed” for one day and then elated the next. You notice several superficial cuts and scars on the patient’s arms and wrists, and he admits to cutting his wrists in order to “feel something other than my emptiness.” Which of the following is the most likely diagnosis for this patient?

Borderline personality disorder

Histrionic personality disorder

A 4-year-old boy presents with language delay, toe-walking, hand-flapping, and intense interest in trains. He makes poor eye contact but occasionally engages in joint attention with his mother when looking at trains. He has sensory sensitivities to loud noises. He can identify all letters and numbers but cannot have reciprocal conversations. His Childhood Autism Rating Scale (CARS) score is borderline. His parents request your opinion on diagnosis and intervention priorities given diagnostic uncertainty and limited early intervention resources in their area.

Presume autism spectrum disorder and implement evidence-based early interventions while monitoring

Diagnose autism spectrum disorder and begin comprehensive ABA therapy immediately

Diagnose language disorder and focus resources on communication training

Refer to genetics for testing before committing to diagnosis or intervention

Delay diagnosis pending further developmental assessment and prioritize speech therapy

Neurobiology of ADHD for USMLE Step 3: High-Yield Psychiatry Lessons

ADHD Pathophysiology - Brain's Busy Wires

Core Deficit: Dysregulation of catecholamine neurotransmission, primarily Dopamine (DA) and Norepinephrine (NE).
Key Brain Circuits & Functions:
- Prefrontal Cortex (PFC): ↓ DA & NE activity impairs executive functions → inattention, hyperactivity, impulsivity.
- Basal Ganglia (Striatum): Altered DA signaling disrupts reward pathways & motivation.
- Limbic System (Amygdala/Cingulate): Involved in emotional dysregulation.
- Cerebellum: Contributes to deficits in timing, coordination, and motor control.

Brain regions and risk genes in ADHD

⭐ Brain imaging studies often show a higher density of dopamine transporters (DAT) in the striatum of individuals with ADHD. This leads to excessive reuptake of dopamine from the synapse, reducing its availability.

Key Neurotransmitters - The Dopamine & NE Show

ADHD pathophysiology centers on catecholamine dysregulation, primarily Dopamine (DA) and Norepinephrine (NE), impacting prefrontal cortex (PFC) function.

Dopamine (DA): The "Motivation Molecule"
- Functions: Governs reward, motivation, and sustained focus.
- Deficit Impact: Leads to impulsivity, poor self-regulation, and a constant search for rewarding stimuli (novelty-seeking).
Norepinephrine (NE): The "Vigilance Vibe"
- Functions: Modulates attention, arousal, vigilance, and executive functions.
- Deficit Impact: Results in inattention, distractibility, and impaired planning/organization.

⭐ Exam Favorite: Structural neuroimaging in ADHD often reveals decreased brain volume in the prefrontal cortex and basal ganglia.

Brain regions and functions relevant to ADHD neurobiology

Neuroanatomy - The Control Network

ADHD involves dysfunction in key brain networks responsible for executive control and attention.

Prefrontal Cortex (PFC): Exhibits ↓ volume and hypoactivation, particularly in the dorsolateral PFC (dlPFC) and anterior cingulate cortex (ACC).
- Clinical link: Correlates with core executive dysfunctions: poor planning, impaired working memory, and weak impulse control.
Basal Ganglia (Striatum): Reduced volume, especially in the caudate nucleus. Dysfunctional fronto-striatal circuits impair dopamine-rich reward pathways.
- Clinical link: Underlies motivation deficits, impulsivity, and hyperactivity.
Cerebellum: Smaller volume, particularly the vermis. Contributes to poor motor coordination and emotional dysregulation.

Brain networks implicated in ADHD

⭐ A key finding is the failure of the PFC to suppress the Default Mode Network (DMN), which is normally active during rest. This results in characteristic attention lapses and mind-wandering.

Neuroimaging Findings - Brain Scan Snapshots

Structural (sMRI):
- ↓ Total cerebral volume & cortical thickness (esp. prefrontal cortex).
- Smaller volumes in key subcortical structures: basal ganglia (caudate, putamen) and corpus callosum.
Functional (fMRI):
- Hypoactivation (↓ activity) in the frontal-striatal networks during executive function & attention tasks.
- Altered connectivity in the Default Mode Network (DMN), linked to attention lapses.

⭐ Delayed cortical maturation is a key finding; the prefrontal cortex, crucial for executive functions, shows a developmental lag of up to 3 years in children with ADHD compared to peers.

fMRI: Hypoactivation in ADHD Prefrontal Cortex

High‑Yield Points - ⚡ Biggest Takeaways

ADHD neurobiology centers on dopamine (DA) and norepinephrine (NE) dysregulation, primarily in the prefrontal cortex (PFC).

Key affected brain circuits involve the fronto-striatal and mesocorticolimbic pathways, impacting executive function and reward processing.

Neuroimaging often shows a smaller brain volume, particularly in the PFC and basal ganglia (specifically the striatum).

A strong genetic component is evident, with high heritability rates for the disorder.

Stimulant medications effectively increase synaptic DA and NE, targeting these core neurochemical imbalances.

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