A 72-year-old man is brought to your office by his daughter due to concern over recent behavioral changes. Over the last several months he has had increasing difficulty with remembering recent events. She mentions that he is embarrassed due to a new inability to control urination. His medical history is significant for hypertension and insomnia. His medications include alprazolam and hydrochlorothiazide. On physical exam, he is oriented to time and place and thinks his daughter is exaggerating; however, when asked to recall 3 items, the patient refuses to continue the mental status exam. He has 5/5 strength bilaterally. He walks in short strides by sliding his feet across the floor. Which of the following would you expect to see in this patient?

Distortion of corona radiata fibers

Depigmentation of the substantia nigra pars compacta

Convex hemorrhage that does not cross suture lines

Atrophy of the caudate and putamen

Atrophy of the subthalamic nucleus

A 37-year-old woman presents to the general medical clinic with a chief complaint of anxiety. She has been having severe anxiety and fatigue for the past seven months. She has difficulty concentrating and her work has suffered, and she has also developed diarrhea from the stress. She doesn't understand why she feels so anxious and is unable to attribute it to any specific aspect of her life right now. You decide to begin pharmacotherapy. All of the following are suitable mechanisms of drugs that can treat this illness EXCEPT:

A drug that blocks dopamine 2 receptors

A drug that stimulates 5-HT1A receptors

A drug that acts as a GABA agonist

A drug that blocks 5-HT reuptake

A drug that blocks both serotonin and norepinephrine reuptake

A patient with a pituitary tumor demonstrates elevated prolactin levels. Which of the following changes in dopamine signaling best explains the hyperprolactinemia?

Decreased tuberoinfundibular dopamine release

Increased D1 receptor activation

Increased D2 receptor activation

A 26-year-old man is brought to the hospital by his wife who complains that her husband has been behaving oddly for the past few hours. The patient’s wife says that she has known him for only 4 months. The wife is unable to give any past medical history. The patient’s speech is difficult to follow, and he seems very distracted. After 15 minutes, he becomes agitated and starts to bang his head on a nearby pillar. He is admitted to the psychiatric ward and is given an emergency medication, after which he calms down. In the next 2 days, he continues to become agitated at times and required 2 more doses of the same drug. On the 4th day of admission, he appears very weak, confused, and does not respond to questions appropriately. His vital signs include: temperature 40.0°C (104.0°F), blood pressure 160/95 mm Hg, and pulse 114/min. On physical examination, he is profusely diaphoretic. He is unable to stand upright or even get up from his bed. Which of the following is the mechanism of action of the drug which most likely caused this patient’s current condition?

Agonistic effect on dopamine receptors

A 28-year-old woman presents with weight gain and a milky-white discharge from her breasts. Patient says she noticed herself gaining weight and a milky white discharge from her breasts. Past medical history is significant for schizophrenia, recently diagnosed and treated with risperidone. No history of headache, nausea, and vomiting. No other current medications. Her last menstrual period was 2 months ago. Review of systems is significant for decreased libido. Patient is afebrile and vital signs are within normal limits. On physical examination, patient had a weight gain of 3 kg (6.6 lb) over the past month. There is bilateral breast tenderness present. A urine pregnancy test is negative. Which of the following is the most likely etiology of this patient’s symptoms?

Decrease in dopamine activity in tuberoinfundibular pathway

Increase in dopamine activity in mesolimbic pathway

Decrease in dopamine activity in mesolimbic pathway

Decrease in dopamine activity in nigrostriatal pathway

Increase in dopamine activity in tuberoinfundibular pathway

A 28-year-old man is brought to the emergency department after he was found half dressed and incoherent in the middle of the road. In the emergency department he states that he has not slept for 36 hours and that he has incredible ideas that will make him a billionaire within a few months. He also states that secret agents from Russia are pursuing him and that he heard one of them speaking through the hospital intercom. His past medical history is significant only for a broken arm at age 13. On presentation, his temperature is 102.2°F (39°C), blood pressure is 139/88 mmHg, pulse is 112/min, and respirations are 17/min. Physical exam reveals pupillary dilation and psychomotor agitation. Which of the following mechanisms is most likely responsible for this patient's symptoms?

Increased biogenic amine release

N-methyl-D-aspartate receptor antagonist

Gamma-aminobutyric acid receptor agonist

A 27-year-old woman is brought to the office at the insistence of her fiancé to be evaluated for auditory hallucinations for the past 8 months. The patient’s fiancé tells the physician that the patient often mentions that she can hear her own thoughts speaking aloud to her. The hallucinations have occurred intermittently for at least 1-month periods. Past medical history is significant for hypertension. Her medications include lisinopril and a daily multivitamin both of which she frequently neglects. She lost her security job 7 months ago after failing to report to work on time. The patient’s vital signs include: blood pressure 132/82 mm Hg; pulse 72/min; respiratory rate 18/min, and temperature 36.7°C (98.1°F). On physical examination, the patient has a flat affect and her focus fluctuates from the window to the door. She is disheveled with a foul smell. She has difficulty focusing on the discussion and does not quite understand what is happening around her. A urine toxicology screen is negative. Which of the following is the correct diagnosis for this patient?

Schizotypal personality disorder

Neurobiological theories of schizophrenia for USMLE Step 3: High-Yield Psychiatry Lessons

Dopamine Hypothesis - The Classic Pathway

Sagittal view of brain showing four key dopamine pathways

Core idea: Schizophrenia symptoms are caused by dysregulated dopamine activity in specific brain circuits.
This hypothesis primarily explains the positive symptoms of schizophrenia.

⭐ The D2 receptor subtype is the main target of typical antipsychotic drugs, which act as antagonists.

📌 Mnemonic: MeLoDramatic (Mesolimbic) for positive symptoms; MeCoN (Mesocortical) for negative symptoms.

Glutamate Hypothesis - NMDA's Downfall

Core Idea: Schizophrenia arises from hypofunction of NMDA glutamate receptors, particularly in the prefrontal cortex (PFC).
Pharmacological Evidence: NMDA antagonists like PCP and ketamine induce both positive and negative symptoms.
Pathway:
- ↓ NMDA activity on GABA interneurons disinhibits cortical glutamate neurons.
- This leads to downstream dopamine dysregulation:
  - Mesocortical Pathway: ↓ Dopamine → Negative symptoms.
  - Mesolimbic Pathway: ↑ Dopamine → Positive symptoms.

⭐ Exam Favorite: Psychosis induced by NMDA antagonists like PCP or ketamine supports the glutamate hypothesis, as they mimic both positive and negative symptoms of schizophrenia.

NMDA Receptor Hypofunction in Schizophrenia

Serotonin & GABA - The Supporting Cast

Serotonin (5-HT) Hypothesis:
- Atypical antipsychotics (e.g., clozapine, risperidone) are potent 5-HT₂A receptor antagonists.
- This action is thought to ↑ dopamine release in the prefrontal cortex, potentially alleviating negative symptoms.
- Psychotomimetics like LSD are 5-HT₂A agonists, supporting this link.
GABAergic Dysfunction:
- Reduced function of GABAergic interneurons, particularly in the prefrontal cortex (PFC).
- Leads to disinhibition of pyramidal neurons, causing cortical noise and cognitive deficits.
- Post-mortem studies show ↓ GABA-producing enzymes (GAD₆₇).

⭐ The efficacy of atypical antipsychotics is tied to their high 5-HT₂A to D₂ receptor blockade ratio, improving negative symptoms and reducing extrapyramidal side effects.

Neuroanatomy - A Shrinking Story

Ventricular Enlargement:
- ↑ size of lateral and third ventricles is a common finding.
- This enlargement is a proxy for surrounding brain tissue loss (atrophy).
Cortical & Subcortical Atrophy:
- Generalized ↓ in gray matter volume, especially in the temporal lobe (hippocampus, amygdala) and frontal lobe.
- Results in widened cortical sulci.
- ↓ thalamic volume, particularly the dorsomedial nucleus.

⭐ Exam Favorite: The hippocampus shows cellular disarray; pyramidal cells are disorganized rather than neatly arranged, contributing to dysfunctional information processing.

High‑Yield Points - ⚡ Biggest Takeaways

Positive symptoms are linked to excess dopamine in the mesolimbic pathway.

Negative symptoms are associated with dopamine hypoactivity in the mesocortical pathway.

Antipsychotic-induced EPS result from dopamine blockade in the nigrostriatal pathway.

Hyperprolactinemia is caused by dopamine blockade in the tuberoinfundibular pathway.

The glutamate hypothesis suggests that NMDA receptor hypofunction also contributes to schizophrenia.

Common structural brain changes include enlarged cerebral ventricles and cortical atrophy.

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