Neuromuscular And Movement Disorders - USMLE Step 2

1. The patient's neuromuscular symptoms (fasciculations and weakness) are primarily due to overstimulation of which type of receptor?
   Answer: Nicotinic acetylcholine receptors This patient has organophosphate poisoning, which inhibits acetylcholinesterase, leading to an excess of acetylcholine. The overstimulation of muscarinic receptors causes the DUMBBELLS symptoms (Diarrhea, Urination, Miosis, Bronchospasm/Bradycardia, Emesis, Lacrimation, Salivation). However, the neuromuscular effects, including fasciculations and subsequent paralytic weakness, are caused by overstimulation of nicotinic acetylcholine receptors at the neuromuscular junction. Alpha-adrenergic and dopamine receptors are not primarily involved in this pathophysiology.
2. Which of the following is the most appropriate initial management for this patient?
   Answer: Oral iron supplementation This patient has restless legs syndrome (RLS). An important secondary cause of RLS is iron deficiency. Before initiating symptomatic treatment with dopamine agonists (e.g., pramipexole) or alpha-2-delta ligands (e.g., gabapentin), it is essential to check iron stores and replete them if low. In patients with RLS and a serum ferritin level <75 ng/mL, oral iron supplementation is recommended as the initial management, as it can significantly improve or resolve symptoms.
3. The patient's neurologic symptoms are most likely caused by antibodies directed against which of the following targets?
   Answer: Presynaptic voltage-gated calcium channels This patient's clinical picture of proximal muscle weakness that improves with activity (Lambert-Eaton facilitation), hyporeflexia, and a history of smoking with a lung mass is highly suggestive of Lambert-Eaton myasthenic syndrome (LEMS), a paraneoplastic syndrome most commonly associated with small cell lung cancer. The pathophysiology of LEMS involves autoantibodies directed against presynaptic P/Q-type voltage-gated calcium channels, which impairs acetylcholine release. Antibodies against postsynaptic ACh receptors cause myasthenia gravis. Antibodies against myelin basic protein are associated with multiple sclerosis, and antibodies against aquaporin-4 are seen in neuromyelitis optica.
4. In addition to glycemic control, which of the following is the most appropriate initial pharmacologic treatment for this patient's symptoms?
   Answer: Amitriptyline This patient has painful diabetic peripheral neuropathy. The first-line treatments for neuropathic pain include tricyclic antidepressants (TCAs) such as amitriptyline, serotonin-norepinephrine reuptake inhibitors (SNRIs) such as duloxetine, and gabapentinoids such as gabapentin or pregabalin. Among the given options, amitriptyline is the most appropriate choice. Opioids like oxycodone are not recommended as first-line therapy due to limited evidence of efficacy and risks of tolerance, dependence, and abuse. NSAIDs like ibuprofen are generally ineffective for neuropathic pain. SSRIs like sertraline are less effective for neuropathic pain compared to TCAs and SNRIs.
5. This patient's clinical findings are most characteristic of which of the following conditions?
   Answer: Amyotrophic lateral sclerosis This patient's presentation with a combination of both upper motor neuron (UMN) signs (hyperreflexia, Babinski signs) and lower motor neuron (LMN) signs (asymmetric weakness, atrophy, fasciculations) is the hallmark of amyotrophic lateral sclerosis (ALS). ALS is a progressive neurodegenerative disease affecting motor neurons. Multiple sclerosis typically presents with sensory deficits and optic neuritis. Myasthenia gravis causes fluctuating weakness without UMN signs. Spinal muscular atrophy is a pure LMN disease and does not cause hyperreflexia or Babinski signs.
6. Which of the following is the most appropriate first-line treatment for this patient's condition?
   Answer: Propranolol This patient has essential tremor, characterized by bilateral action (postural and kinetic) tremor that improves with alcohol and has a positive family history. The two first-line pharmacologic treatments for essential tremor are propranolol (a non-selective beta-blocker) and primidone (an anticonvulsant). Among the choices given, propranolol is the most appropriate. Levodopa is used for Parkinson disease, which typically presents with resting tremor, bradykinesia, and rigidity. Gabapentin may have some efficacy but is considered a second-line agent. Clonazepam can be helpful but is also not first-line due to concerns about sedation and dependence.
7. Which of the following is the most likely diagnosis?
   Answer: Polymyositis This patient's presentation of symmetric proximal muscle weakness, a markedly elevated creatine kinase (CK) level, and a positive anti-Jo-1 antibody is classic for polymyositis, an idiopathic inflammatory myopathy. The absence of facial/ocular weakness and the high CK level differentiate it from neuromuscular junction disorders like myasthenia gravis or LEMS, where CK is typically normal. Amyotrophic lateral sclerosis involves both upper and lower motor neuron signs, which are absent here.
8. Which of the following is the most accurate test to confirm the diagnosis?
   Answer: Nerve conduction studies The patient's symptoms are classic for carpal tunnel syndrome, which is caused by compression of the median nerve at the wrist. While the diagnosis is often made clinically, nerve conduction studies (NCS) are the most accurate diagnostic test. NCS can confirm the diagnosis, determine the severity of nerve compression, and rule out other neuropathies. Wrist X-rays are not useful unless a fracture is suspected. MRI of the cervical spine would evaluate for cervical radiculopathy, which can present similarly but typically involves neck pain and different sensory distribution. TSH is checked to rule out hypothyroidism as a secondary cause but does not confirm the diagnosis of carpal tunnel syndrome itself.
9. What is the most appropriate next step in the management of this patient?
   Answer: Administer intravenous immunoglobulins This patient's presentation of acute, ascending, symmetric paralysis with areflexia following a recent gastrointestinal infection is characteristic of Guillain-Barré syndrome (GBS). While her vital capacity of 25 mL/kg is concerning and requires close respiratory monitoring, she does not yet require intubation (typically indicated when vital capacity drops below 15-20 mL/kg or with other signs of respiratory compromise). The most appropriate next step is to initiate disease-modifying treatment with intravenous immunoglobulins (IVIG) or plasmapheresis, both of which are equally effective first-line treatments that can halt disease progression. Diagnostic tests like MRI and lumbar puncture can be useful but should not delay treatment in a clinically obvious case.
10. Which of the following is the most appropriate initial test to confirm the diagnosis?
    Answer: Acetylcholine receptor antibody assay This patient's presentation of fluctuating ptosis, diplopia, and bulbar weakness that worsens with activity is classic for myasthenia gravis (MG). The most appropriate initial test is serologic testing for acetylcholine receptor (AChR) antibodies, which is positive in about 85% of patients with generalized MG and has high specificity. If AChR antibodies are negative, testing for MuSK antibodies should be performed. Repetitive nerve stimulation studies show a decremental response in MG but are less specific and typically used when antibody testing is negative or to confirm the diagnosis functionally. Single-fiber EMG is the most sensitive test but is technically demanding and reserved for cases where other tests are inconclusive. CT scan of the chest to screen for thymoma is important but performed after confirming the diagnosis.
11. These findings are most consistent with a lesion affecting which of the following structures?
    Answer: Anterior horn cells The patient's clinical presentation consists exclusively of lower motor neuron (LMN) signs: weakness, muscle atrophy, fasciculations, flaccid tone, and areflexia. The anterior horn cells of the spinal cord contain the cell bodies of LMNs. Diseases affecting these cells, such as spinal muscular atrophy or the LMN component of ALS, would produce these findings. Lesions of the corticospinal tracts (upper motor neurons) would cause spasticity and hyperreflexia. Posterior column lesions cause loss of vibration and proprioception. Basal ganglia dysfunction causes movement disorders like Parkinsonism or chorea.
12. The involuntary movements are best described as which of the following?
    Answer: Peak-dose dyskinesia This patient is experiencing motor complications of long-term levodopa therapy. The return of parkinsonian symptoms before the next dose is known as 'wearing-off.' The involuntary writhing, choreiform movements that occur when the medication effect is at its maximum are termed peak-dose dyskinesias. These are caused by excessive dopaminergic stimulation in a sensitized striatum. Off-period dystonia occurs when levodopa levels are low. Akathisia is a sense of restlessness. Myoclonus is characterized by brief, shock-like muscle jerks.
13. Which of the following is the most effective treatment for this patient's condition?
    Answer: Botulinum toxin injections This patient has cervical dystonia (spasmodic torticollis), a form of focal dystonia characterized by sustained muscle contractions causing abnormal neck posturing. The ability to temporarily correct the posture with a sensory trick ('geste antagoniste') is a classic feature. The most effective and first-line treatment for focal dystonias is intramuscular injection of botulinum toxin into the affected muscles (e.g., sternocleidomastoid, trapezius). This selectively weakens the overactive muscles, relieving the abnormal posture and pain. Oral medications like baclofen are less effective and have more systemic side effects. IVIG is not indicated. Physical therapy can be a helpful adjunct but is not the primary treatment.
14. Genetic testing in this patient is most likely to reveal which of the following findings?
    Answer: An expanded number of CAG trinucleotide repeats The clinical presentation of chorea, psychiatric changes (irritability, impulsivity), and a positive family history is highly suggestive of Huntington disease. This is an autosomal dominant neurodegenerative disorder caused by an expanded number of CAG (cytosine-adenine-guanine) trinucleotide repeats in the huntingtin gene. A point mutation in the copper-transporting ATPase gene (ATP7B) causes Wilson disease. A deletion in the SMN1 gene causes spinal muscular atrophy. A GGGGCC repeat expansion in the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia.
15. Which of the following is the most appropriate initial medication for this patient?
    Answer: Carbidopa-levodopa This patient presents with the classic triad of Parkinson disease: bradykinesia (slowness, soft voice, small handwriting), resting tremor, and rigidity. Given his age (>65) and significant functional impairment (falls), carbidopa-levodopa is the most effective and appropriate initial treatment. Dopamine agonists like pramipexole are often used in younger patients to delay levodopa-related motor complications but are less effective and have a higher risk of side effects (e.g., hallucinations) in older adults. Benztropine is an anticholinergic used primarily for tremor in younger patients and is poorly tolerated in the elderly. Amantadine offers mild symptomatic benefit but is less potent than levodopa.
16. What is the most appropriate next step in management?
    Answer: Perform endotracheal intubation This patient is in myasthenic crisis, a life-threatening exacerbation of myasthenia gravis characterized by respiratory muscle weakness leading to respiratory failure. Despite normal oxygenation and PaCO2, the clinical signs of respiratory distress (accessory muscle use) and the severely reduced negative inspiratory force (NIF <-20 cm H2O is an indication) signify impending respiratory collapse. The most appropriate next step is to secure the airway with endotracheal intubation and initiate mechanical ventilation. Edrophonium is a diagnostic tool and not used for management. Increasing pyridostigmine can worsen secretions and is not sufficient for respiratory failure. While the crisis was likely triggered by an infection, securing the airway is the immediate priority.
17. In addition to respiratory status, this patient should be closely monitored for which of the following complications?
    Answer: Autonomic dysfunction Guillain-Barré syndrome is an inflammatory demyelinating polyneuropathy that affects not only motor nerves but also autonomic nerves. Autonomic dysfunction is a common and potentially life-threatening complication. Manifestations can include cardiac arrhythmias (tachycardia, bradycardia), blood pressure instability (hypertension or hypotension), urinary retention, and ileus. Therefore, patients require continuous cardiac and blood pressure monitoring. Increased intracranial pressure, seizures, and cognitive decline are not typical complications of GBS.
18. These physical examination findings are characteristic of damage to which of the following?
    Answer: Upper motor neurons The patient's findings of spasticity (increased tone), hyperreflexia, and a positive Babinski sign (upgoing great toe) are classic signs of an upper motor neuron (UMN) lesion. The internal capsule contains corticospinal tracts, which are UMN pathways. Lower motor neuron lesions cause flaccid paralysis, atrophy, fasciculations, and hyporeflexia. Neuromuscular junction disorders cause fluctuating weakness without reflex changes. Peripheral sensory nerve damage causes sensory loss.

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