Q-Bank Breakdown: Parkinson disease — Why Every Answer Choice Matters

You just missed a Parkinson disease question and it felt “too easy”… until you saw the explanation and realized every distractor was booby-trapped with a different movement disorder. These are the questions that separate pattern-recognition from real test-day confidence. Let’s do a Q-bank-style breakdown where every answer choice matters.

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Clinical vignette (Q-bank style)

A 66-year-old man comes to clinic for progressively worsening slowness and difficulty with daily tasks over 1 year. His wife reports he has a reduced facial expression and a “soft voice.” He has constipation and has been acting out his dreams at night. On exam, he has a resting tremor in the right hand described as “pill-rolling,” cogwheel rigidity, and bradykinesia. Gait is slow with shuffling steps and reduced arm swing on the right.

Which of the following is the most likely underlying pathology?

A. Loss of dopaminergic neurons in the substantia nigra pars compacta with Lewy bodies
B. Demyelination of the central nervous system with periventricular plaques
C. Degeneration of caudate nucleus and putamen with CAG repeat expansion
D. Autoantibodies against postsynaptic acetylcholine receptors at the neuromuscular junction
E. Loss of motor neurons in anterior horn cells with corticospinal tract degeneration

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How to spot Parkinson disease fast (without rushing)

This stem is screaming Parkinson disease:

• TRAP features:
  • Tremor at rest (pill-rolling)
  • Rigidity (cogwheel/lead-pipe)
  • Akinesia/bradykinesia
  • Postural instability (often later)
• Shuffling gait, reduced arm swing, masked facies, hypophonia
• Non-motor clues that show up on USMLE:
  • Constipation (autonomic dysfunction)
  • REM sleep behavior disorder (acting out dreams)
  • Depression/anxiety/apathy
  • Anosmia can precede motor symptoms

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Correct answer: A. Loss of dopaminergic neurons in substantia nigra pars compacta with Lewy bodies

What’s happening pathologically?

• Degeneration of dopaminergic neurons in substantia nigra pars compacta (SNc)
• Leads to decreased dopamine in the striatum (caudate + putamen)
• Classic inclusion: Lewy bodies (intracytoplasmic eosinophilic inclusions made of $\alpha$-synuclein)

High-yield basal ganglia physiology (Step 1 favorite)

Dopamine from SNc:

• Stimulates the direct pathway via D1 $\rightarrow$ promotes movement
• Inhibits the indirect pathway via D2 $\rightarrow$ also promotes movement

Net effect of dopamine = pro-movement.
So dopamine loss $\rightarrow$ hypokinetic state (bradykinesia, rigidity).

Meds + complications (Step 2 / clinical management)

• Carbidopa/levodopa for symptomatic control (best for bradykinesia/rigidity)
  • Carbidopa inhibits peripheral DOPA decarboxylase $\rightarrow$ more levodopa reaches CNS + fewer peripheral side effects
• Adverse effects to remember: nausea, orthostasis, hallucinations, dyskinesias
• Adjuncts:
  • MAO-B inhibitors (selegiline, rasagiline)
  • COMT inhibitors (entacapone)
  • Dopamine agonists (pramipexole, ropinirole)
• Deep brain stimulation (STN or GPi) for medication-refractory motor fluctuations/tremor

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Why the other answer choices are wrong (and what they’re trying to trick you into)

B. Demyelination of the CNS with periventricular plaques

This is Multiple Sclerosis (MS).

How it would look instead:

• Neurologic deficits separated by time and space (e.g., optic neuritis, sensory loss, weakness)
• Often younger adults; women > men
• Internuclear ophthalmoplegia, Lhermitte sign
• MRI: periventricular plaques (“Dawson fingers”)

Why it’s not Parkinson disease:

• Parkinson disease is a neurodegenerative movement disorder, not a demyelinating process
• MS symptoms aren’t typically a classic resting tremor + rigidity/bradykinesia syndrome

USMLE pearl: MS is associated with oligoclonal bands in CSF (IgG) and immune-mediated demyelination.

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C. Degeneration of caudate nucleus and putamen with CAG repeat expansion

This is Huntington disease.

How it would look instead:

• Chorea (hyperkinetic movements), dystonia
• Psychiatric changes (irritability, depression), cognitive decline
• Onset often 30–50 years (but variable)
• Autosomal dominant CAG repeat expansion (anticipation, paternal transmission commonly emphasized)

Why it’s not Parkinson disease:

• Huntington is typically hyperkinetic, while Parkinson is hypokinetic
• Parkinson features bradykinesia/rigidity and resting tremor, not chorea

USMLE pearl: Loss of GABAergic neurons in striatum $\rightarrow$ increased dopamine relative activity; imaging may show caudate atrophy with enlarged lateral ventricles.

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D. Autoantibodies against postsynaptic acetylcholine receptors at the neuromuscular junction

This is Myasthenia gravis (MG).

How it would look instead:

• Fluctuating weakness that worsens with use
• Ptosis, diplopia, dysphagia
• Normal sensation and reflexes
• Improved with rest and acetylcholinesterase inhibitors (e.g., pyridostigmine)

Why it’s not Parkinson disease:

• Parkinson is a central movement disorder; MG is a neuromuscular junction transmission problem
• Parkinson causes bradykinesia/rigidity and gait changes—not fatigable ocular/bulbar weakness

USMLE pearl: MG is associated with thymoma/thymic hyperplasia; treat exacerbations with IVIG or plasmapheresis.

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E. Loss of motor neurons in anterior horn cells with corticospinal tract degeneration

This is Amyotrophic lateral sclerosis (ALS).

How it would look instead:

• Both UMN and LMN signs:
  • UMN: hyperreflexia, spasticity, Babinski
  • LMN: fasciculations, atrophy, weakness
• No sensory loss (classic teaching)
• Progressive, can cause respiratory failure

Why it’s not Parkinson disease:

• Parkinson is primarily extrapyramidal (basal ganglia) with tremor/rigidity/bradykinesia
• ALS does not present with resting tremor or classic shuffling gait/rigidity syndrome

USMLE pearl: Treat ALS with riluzole (glutamate inhibitor) and edaravone (free radical scavenger)—modest disease progression benefit.

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Rapid differentiation table (high-yield)

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Test-day takeaways (what to lock in)

• Parkinson disease = TRAP + shuffling gait + masked facies/hypophonia, often with non-motor prodrome (constipation, REM sleep behavior disorder, anosmia).
• Pathology: SNc dopaminergic neuron loss + Lewy bodies containing $\alpha$-synuclein.
• When a distractor mentions:
  • Periventricular plaques $\rightarrow$ think MS
  • CAG repeats + caudate/putamen $\rightarrow$ think Huntington
  • AChR antibodies $\rightarrow$ think Myasthenia gravis
  • UMN + LMN $\rightarrow$ think ALS