USMLE Step 1 & 2 Movement Disorders (Parkinson's, Huntington's)
Last updated: May 2, 2026
Movement Disorders (Parkinson's, Huntington's) questions are one of the highest-leverage areas to study for the USMLE Step 1 & 2. This guide breaks down the rule, the elements you need to recognize, the named traps that catch most students, and a memory aid that scales to test day. Read it once, then practice the same sub-topic adaptively in the app.
The rule
Movement disorders split cleanly into hypokinetic (too little movement, rigid, slow) and hyperkinetic (too much movement, choreiform, ballistic). Parkinson's disease is the prototype hypokinetic disorder, driven by loss of dopaminergic neurons in the substantia nigra pars compacta, producing the TRAP tetrad — Tremor (resting), Rigidity (cogwheel), Akinesia/bradykinesia, Postural instability. Huntington's disease is the prototype hyperkinetic disorder, an autosomal-dominant CAG trinucleotide repeat expansion in the HTT gene on chromosome 4 producing chorea, behavioral changes, and dementia from caudate (and later putamen) atrophy. The basal-ganglia circuit logic is symmetric: PD loses the direct pathway's dopaminergic drive (less movement); HD loses GABAergic medium spiny neurons of the indirect pathway (less inhibition, so more movement).
Elements breakdown
Parkinson's disease — clinical features
Hypokinetic disorder from substantia nigra pars compacta dopaminergic neuron loss with Lewy bodies (alpha-synuclein aggregates).
- Resting pill-rolling tremor, 4-6 Hz
- Cogwheel rigidity on passive movement
- Bradykinesia, masked facies, micrographia
- Shuffling gait, postural instability later
- Asymmetric onset, anosmia, REM sleep behavior disorder prodrome
Huntington's disease — clinical features
Autosomal-dominant CAG repeat expansion (>36) in HTT gene; anticipation through paternal transmission.
- Chorea: brief, irregular, non-stereotyped movements
- Behavioral changes: irritability, depression, psychosis
- Progressive dementia, executive dysfunction
- Onset typically age 30-50
- Caudate atrophy on MRI — boxcar ventricles
Common examples:
- Family history of suicide or early dementia
- Patient appears 'fidgety' on exam
Basal ganglia circuitry
Direct pathway promotes movement (D1, excitatory to thalamocortical drive); indirect pathway inhibits movement (D2, net inhibitory).
- PD: SNc dopamine loss → less direct drive, more indirect → bradykinesia
- HD: caudate medium spiny GABA neurons (indirect) lost → disinhibition → chorea
- Both converge on thalamocortical output abnormalities
- STN deep brain stimulation helps PD by reducing indirect overactivity
Pharmacotherapy — Parkinson's
Aim is to restore striatal dopamine signaling or block compensatory cholinergic overactivity.
- Levodopa/carbidopa — most effective; on-off and dyskinesia long-term
- Dopamine agonists (pramipexole, ropinirole) — first-line in younger patients
- MAO-B inhibitors (selegiline, rasagiline) — mild, neuroprotective claim
- COMT inhibitors (entacapone) — extend levodopa
- Amantadine — mild, helps levodopa-induced dyskinesia
- Anticholinergics (benztropine, trihexyphenidyl) — tremor-predominant, young
Pharmacotherapy — Huntington's
Symptomatic only; no disease-modifying therapy.
- Tetrabenazine or deutetrabenazine — VMAT2 inhibitors, deplete presynaptic dopamine, reduce chorea
- Antipsychotics (haloperidol, risperidone) — chorea + psychiatric symptoms
- SSRIs for depression
- Genetic counseling — anticipation, 50% offspring risk
Other movement disorder mimics to know
Common close-mimic distractors that test recognition of cardinal features.
- Essential tremor: action/postural, bilateral, alcohol-responsive, beta-blockers/primidone
- Wilson disease: young patient, Kayser-Fleischer rings, low ceruloplasmin, wing-beating tremor
- Drug-induced parkinsonism: antipsychotics, metoclopramide; symmetric, no Lewy bodies
- Lewy body dementia: parkinsonism + early dementia + visual hallucinations + REM sleep behavior disorder
- Progressive supranuclear palsy: vertical gaze palsy, axial rigidity, early falls
- Sydenham chorea: post-streptococcal, children, self-limited
Common patterns and traps
The TRAP Tetrad Anchor
USMLE vignettes for Parkinson's almost always plant at least two of the four TRAP features in the stem, often with one feature stated obliquely (micrographia for bradykinesia, masked facies hidden in 'expressionless face'). Recognizing two TRAP features should immediately commit you to PD before reading the answer choices. The trap is being seduced by a single feature like 'tremor' that fits essential tremor better.
A correct answer choice names the substantia nigra pars compacta, alpha-synuclein/Lewy bodies, or levodopa; an incorrect choice names the cerebellum, beta-blockers, or copper metabolism.
The CAG Anticipation Family Tree
Huntington's stems often hide the diagnosis in the family history — a father who died in his 40s 'in a state hospital,' a paternal grandfather with 'early Alzheimer's,' or a sibling who developed 'psychiatric problems' young. Anticipation (earlier onset and worse severity in successive generations) is dramatically worse with paternal transmission because of CAG repeat instability in spermatogenesis. The trap is missing that the 'depression' or 'psychiatric admission' in the family is actually undiagnosed HD.
A correct answer names CAG trinucleotide repeat expansion, chromosome 4, or caudate atrophy; an incorrect choice names CGG (fragile X), CTG (myotonic dystrophy), or GAA (Friedreich ataxia).
The Tremor Character Swap
The single most common close-mimic trap in this domain is essential tremor masquerading as Parkinson's. Essential tremor is a postural/action tremor that worsens with reaching, is bilateral and symmetric, often improves with alcohol, and has a positive family history; Parkinson's tremor is at rest, asymmetric, and disappears with purposeful movement. The exam swaps one descriptor — 'tremor while reaching for the cup' vs 'tremor while sitting still' — and the diagnosis flips.
Distractor mentions propranolol, primidone, or 'improves with two glasses of wine'; the correct PD choice mentions levodopa, dopamine agonists, or asymmetric onset.
The Drug-Induced Parkinsonism Distractor
Patients on antipsychotics (haloperidol, risperidone), antiemetics (metoclopramide, prochlorperazine), or even valproate can develop a parkinsonian syndrome that is symmetric, lacks Lewy bodies, and remits after the offending drug is stopped. The exam plants a recent prescription in the medication list and expects you to identify the drug rather than start levodopa. Missing this is a classic 'next-best-step' failure.
The vignette mentions a recent psychiatric hospitalization and a new antipsychotic; the correct answer is to discontinue the offending agent, not to start levodopa.
The Wilson Disease Mimic
In any patient under 40 with movement abnormalities, Wilson disease must be on the differential — especially if there are hepatic findings, psychiatric symptoms, or Kayser-Fleischer rings on slit-lamp. The tremor is classically a coarse 'wing-beating' tremor with proximal flapping. The exam plants a young adult with mixed neuropsychiatric and hepatic findings and rewards low ceruloplasmin / high urinary copper as the answer.
Vignette mentions elevated transaminases, a brown corneal ring, or a teenage sibling with cirrhosis; the answer is serum ceruloplasmin or 24-hour urinary copper, not genetic testing for HTT.
How it works
Picture Mr. Alvarado, 64, with a 2-year history of a right-hand resting tremor that disappears when he reaches for his coffee mug, plus a stooped posture and shrinking handwriting. The asymmetric resting tremor and bradykinesia point you to Parkinson's disease, and you anchor on dopaminergic neuron loss in the substantia nigra pars compacta. Now contrast Ms. Reyes, 42, brought in by her husband for two years of fidgety finger movements she dismisses as 'just nerves,' increasing irritability, and a father who 'died young in a psychiatric hospital.' That triad — chorea, behavior change, family history with anticipation — is the Huntington's signature, and you'd expect caudate atrophy on MRI. The exam loves to pair the same vignette spine (movement disorder, family member affected) with a swap of one detail — resting vs action tremor, asymmetric vs bilateral, age 60 vs age 40 — that flips the diagnosis. Lock the cardinal features and the inheritance pattern, and the rest of the differential collapses around them.
Worked examples
Which of the following best explains this patient's clinical findings?
- A Loss of dopaminergic neurons in the substantia nigra pars compacta with intracytoplasmic alpha-synuclein aggregates ✓ Correct
- B Autosomal-dominant CAG trinucleotide repeat expansion in the HTT gene
- C Cerebellar hemisphere degeneration with loss of Purkinje cells
- D D2 dopamine receptor blockade from a recently introduced medication
Why A is correct: The asymmetric resting pill-rolling tremor that disappears with purposeful movement, cogwheel rigidity, bradykinesia (micrographia, slow buttoning), reduced arm swing, and masked facies are the TRAP tetrad of Parkinson's disease. The pathologic substrate is loss of dopaminergic neurons in the substantia nigra pars compacta with Lewy bodies (alpha-synuclein aggregates).
Why each wrong choice fails:
- B: CAG expansion in HTT causes Huntington's disease, which is hyperkinetic (chorea), typically presents in the 30s-50s with behavioral and cognitive changes, and would not produce a resting tremor or cogwheel rigidity. (The CAG Anticipation Family Tree)
- C: Cerebellar Purkinje cell loss produces ataxia, intention tremor (worsens approaching target), and dysmetria, not a resting tremor with rigidity and bradykinesia. (The Tremor Character Swap)
- D: Drug-induced parkinsonism is symmetric and follows a recent antipsychotic, antiemetic, or similar dopamine-blocking agent. This patient takes only atorvastatin and aspirin, neither of which causes parkinsonism, and his presentation is asymmetric. (The Drug-Induced Parkinsonism Distractor)
Which of the following genetic abnormalities is most likely responsible for this patient's condition?
- A CGG trinucleotide repeat expansion on the X chromosome
- B CAG trinucleotide repeat expansion on chromosome 4 ✓ Correct
- C GAA trinucleotide repeat expansion on chromosome 9
- D Point mutation in the ATP7B gene on chromosome 13
Why B is correct: Chorea with motor impersistence (inability to sustain tongue protrusion, milkmaid grip), behavioral and cognitive decline beginning in the fourth decade, paternal family history suggesting anticipation, and caudate atrophy with boxcar ventricles on MRI are diagnostic of Huntington's disease — an autosomal-dominant CAG trinucleotide repeat expansion (>36 repeats) in the HTT gene on chromosome 4.
Why each wrong choice fails:
- A: CGG expansion in FMR1 on the X chromosome causes Fragile X syndrome, which presents in childhood with intellectual disability, macroorchidism, and long facies — not adult-onset chorea with caudate atrophy. (The CAG Anticipation Family Tree)
- C: GAA expansion in the frataxin gene on chromosome 9 causes Friedreich ataxia, which presents in childhood/adolescence with cerebellar ataxia, hypertrophic cardiomyopathy, and diabetes — not chorea with caudate atrophy. (The CAG Anticipation Family Tree)
- D: ATP7B mutations cause Wilson disease, which can present in young adults with movement abnormalities and psychiatric symptoms but features Kayser-Fleischer rings, low ceruloplasmin, hepatic dysfunction, and a wing-beating (not choreiform) tremor — and would not produce caudate atrophy with boxcar ventricles. (The Wilson Disease Mimic)
Which of the following is the most appropriate initial pharmacologic therapy for this patient?
- A Levodopa-carbidopa
- B Pramipexole ✓ Correct
- C Tetrabenazine
- D Haloperidol
Why B is correct: In younger patients (<65-70) with mild Parkinson's disease and preserved cognition, dopamine agonists such as pramipexole or ropinirole are commonly preferred initial therapy to delay levodopa exposure and the eventual on-off fluctuations and dyskinesias that complicate long-term levodopa use. Levodopa is reserved for older patients or those with more functionally limiting symptoms.
Why each wrong choice fails:
- A: Levodopa-carbidopa is the most effective antiparkinsonian drug overall but is typically deferred in younger functional patients to delay motor complications (dyskinesias, on-off phenomena). It would be the first-line choice in an older patient (>70) or with more disabling disease, but not here. (The TRAP Tetrad Anchor)
- C: Tetrabenazine is a VMAT2 inhibitor used to suppress chorea in Huntington's disease by depleting presynaptic dopamine. Giving it to a Parkinson's patient would worsen bradykinesia by further depleting the already-deficient striatal dopamine. (The CAG Anticipation Family Tree)
- D: Haloperidol is a high-potency D2 antagonist used for psychosis and chorea. In a Parkinson's patient it would precipitate severe drug-induced parkinsonism by blocking the few remaining dopaminergic pathways and is contraindicated. (The Drug-Induced Parkinsonism Distractor)
Memory aid
Parkinson's = TRAP (Tremor at rest, Rigidity, Akinesia, Postural instability). Huntington's = the 3 D's: Dance (chorea), Dementia, Depression — caused by CAG repeats on chromosome 4 ('Caudate Atrophy in Generation 4').
Key distinction
Resting tremor that disappears with action + asymmetric onset = Parkinson's. Choreiform fidgety movements + family history + behavioral change = Huntington's. The tremor character (rest vs action) and the family history are the two highest-yield discriminators on the exam.
Summary
Parkinson's is hypokinetic (substantia nigra dopamine loss, TRAP, levodopa); Huntington's is hyperkinetic (autosomal-dominant CAG expansion, caudate atrophy, chorea + dementia + behavior change, tetrabenazine).
Practice movement disorders (parkinson's, huntington's) adaptively
Reading the rule is the start. Working USMLE Step 1 & 2-format questions on this sub-topic with adaptive selection, watching your mastery score climb in real time, and seeing the items you missed return on a spaced-repetition schedule — that's where score lift actually happens. Free for seven days. No credit card required.
Start your free 7-day trialFrequently asked questions
What is movement disorders (parkinson's, huntington's) on the USMLE Step 1 & 2?
Movement disorders split cleanly into hypokinetic (too little movement, rigid, slow) and hyperkinetic (too much movement, choreiform, ballistic). Parkinson's disease is the prototype hypokinetic disorder, driven by loss of dopaminergic neurons in the substantia nigra pars compacta, producing the TRAP tetrad — Tremor (resting), Rigidity (cogwheel), Akinesia/bradykinesia, Postural instability. Huntington's disease is the prototype hyperkinetic disorder, an autosomal-dominant CAG trinucleotide repeat expansion in the HTT gene on chromosome 4 producing chorea, behavioral changes, and dementia from caudate (and later putamen) atrophy. The basal-ganglia circuit logic is symmetric: PD loses the direct pathway's dopaminergic drive (less movement); HD loses GABAergic medium spiny neurons of the indirect pathway (less inhibition, so more movement).
How do I practice movement disorders (parkinson's, huntington's) questions?
The fastest way to improve on movement disorders (parkinson's, huntington's) is targeted, adaptive practice — working questions that focus on your specific weak spots within this sub-topic, getting immediate feedback, and revisiting items you missed on a spaced-repetition schedule. Neureto's adaptive engine does this automatically across the USMLE Step 1 & 2; start a free 7-day trial to see your sub-topic mastery climb in real time.
What's the most important distinction to remember for movement disorders (parkinson's, huntington's)?
Resting tremor that disappears with action + asymmetric onset = Parkinson's. Choreiform fidgety movements + family history + behavioral change = Huntington's. The tremor character (rest vs action) and the family history are the two highest-yield discriminators on the exam.
Is there a memory aid for movement disorders (parkinson's, huntington's) questions?
Parkinson's = TRAP (Tremor at rest, Rigidity, Akinesia, Postural instability). Huntington's = the 3 D's: Dance (chorea), Dementia, Depression — caused by CAG repeats on chromosome 4 ('Caudate Atrophy in Generation 4').
What's a common trap on movement disorders (parkinson's, huntington's) questions?
Mistaking essential tremor (action, bilateral, alcohol-responsive) for Parkinson's resting tremor
What's a common trap on movement disorders (parkinson's, huntington's) questions?
Forgetting that Huntington's anticipation is worse with paternal transmission
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