USMLE PRACTICE QUESTION #113

TOPIC: Neuropathology/Diagnosis

Dec 25, 2024

In USMLE practice question #113, we’ll assess our ability to recognize the imaging findings of a commonly tested neurological condition.

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A 28-year-old female presents to the neurologist with difficulty walking and occasional episodes of muscle weakness and spasticity. She reports transient episodes of visual blurriness and has a history of "pins and needles" sensations in her legs. Neurological examination reveals spasticity and hyperreflexia in both lower extremities. Which of the following is the most sensitive test for confirming the diagnosis?

A) Evoked potentials
B) Cerebrospinal fluid analysis for oligoclonal bands
C) MRI of the brain and spinal cord
D) Anti-myelin oligodendrocyte glycoprotein antibody testing
E) Serum anti-nuclear antibodies

Detailed Breakdown of Answers + Correct Answer Below ⏬

ANSWER + QUESTION BREAKDOWN

It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:

Step 1. Read the last line to get to the heart of the question: “Which of the following is the most sensitive test for confirming the diagnosis?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 2n order. 1st: Diagnose the condition; 2nd: Identify the most sensitive confirmatory test.

Step 3: Read the vignette carefully and ask yourself: “Based on my suspected diagnosis, which test is most sensitive for making a diagnosis?”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

This vignette describes a 28-year-old female with symptoms highly suggestive of multiple sclerosis.

ANSWER CHOICES:

CHOICE A: Evoked potentials

Explanation: Evoked potentials measure the brain's electrical activity in response to stimuli (visual, auditory, or somatosensory). Delayed responses can indicate demyelination. While evoked potentials can help detect demyelination, they are not the most sensitive diagnostic test for the suspected diagnosis.

CHOICE B: Cerebrospinal fluid analysis for oligoclonal bands

Explanation: Oligoclonal bands in cerebrospinal fluid indicate inflammation within the CNS and are commonly found in patients with MS. Although oligoclonal bands are useful in supporting the diagnosis of MS, they are not as sensitive as the correct answer.

CHOICE C: MRI of the brain and spinal cord

Explanation: MRI is the most sensitive imaging modality for detecting demyelinating lesions in multiple sclerosis. It can show characteristic white matter plaques in both the brain and spinal cord.

CHOICE D: Anti-myelin oligodendrocyte glycoprotein antibody testing

Explanation: Anti-myelin oligodendrocyte glycoprotein (MOG) antibodies are associated with MOG antibody-associated disease (MOGAD), which can present similarly to MS but is a distinct condition. Since MOGAD is a separate condition, testing for MOG antibodies would not be appropriate for diagnosing classic multiple sclerosis.

CHOICE E: Serum anti-nuclear antibodies

Explanation: Anti-nuclear antibodies (ANA) are typically used to diagnose autoimmune diseases such as systemic lupus erythematosus (SLE). ANA testing is not relevant to diagnosing multiple sclerosis.

FINAL VERDICT…

The most sensitive test for confirming a diagnosis of multiple sclerosis is an MRI of the brain and spinal cord, which can reveal characteristic demyelinating lesions (plaques). These plaques are typically found in white matter regions such as periventricular areas, juxtacortical regions, and the spinal cord. MRI is highly sensitive for detecting both symptomatic and asymptomatic lesions, making it the gold standard diagnostic tool for MS.

While other tests—such as cerebrospinal fluid analysis for oligoclonal bands or evoked potentials—can support an MS diagnosis, MRI remains the most sensitive initial test for detecting demyelination.

KEY CONCEPTS:

Multiple sclerosis (MS) is an autoimmune disease characterized by demyelination in the CNS, leading to a wide variety of neurological symptoms, including motor weakness, sensory disturbances, and visual problems.
The most sensitive test for diagnosing MS is MRI of the brain and spinal cord, which can reveal characteristic lesions (plaques) in the white matter, particularly in periventricular areas, the spinal cord, and the optic nerves.
Other diagnostic tools include cerebrospinal fluid (CSF) analysis for oligoclonal bands, which indicate inflammation in the CNS, and evoked potentials, which assess nerve conduction.

CORRECT ANSWER: C) MRI of the brain and spinal cord