USMLE Step 1 Question
TOPIC: MSK
The following is a high-yield USMLE Step 1 exam question.
To get the most out of this practice question deep-dive, I recommend doing the following:
Go through the question using our test-taking strategy (read about it HERE)
Formulate your answer and find it in the answer choices
If you believe you’ve identified the correct answer, you can scroll to the bottom of the post to see if you’re right; if you are - congrats! Be sure to read through the deep-dive section to extract more knowledge from the question
If you aren’t 100% sure about the answer, start with the deep-dive section to get clear on the concept(s), then try to formulate an answer from there; once you believe you’ve found the answer, scroll to the bottom of the page (if you were wrong, re-read the deep-dive to figure out why).
QUESTION:
A 35-year-old woman presents to her primary care physician with pain over the radial side of her wrist, exacerbated by thumb extension and ulnar deviation. She is an avid tennis player who exercises regularly and maintains a healthy BMI. Physical examination reveals pain when the thumb is extended against resistance with the write in a neutral position. Her temperature is 98.6°F, pulse is 75/minute, respirations are 14/minute, and blood pressure is 115/80 mmHg. Which of the following anatomical structures is most directly involved in the pathogenesis of this patient's condition?
A. Extensor pollicis longus tendon
B. Abductor pollicis longus tendon
C. Flexor pollicis longus tendon
D. Extensor pollicis brevis tendon
E. Flexor pollicis brevis tendon
To jump to the answer, scroll to the bottom of the post. If you’re unsure, here’s a DEEP-DIVE into each answer choice.
DEEP-DIVE:
Given the specific details of the question, particularly the pain on thumb extension against resistance with the wrist in a neutral position, the focus is on the tendons responsible for extending the thumb. This narrows down the likely involved structures to those associated with thumb extension, mainly the extensor tendons. Let’s take a closer look at our option choices and dive into the important details that gives us a better understanding of each.
A) Extensor Pollicis Longus Tendon
Associated Muscle: Extensor Pollicis Longus
Nerve Innervation: Radial nerve (specifically, the posterior interosseous nerve, a branch of the radial nerve)
Origin: Middle third of the posterior surface of the ulna and the interosseous membrane
Insertion: Base of the distal phalanx of the thumb
Relevance: Though it extends the thumb, its action and the location of the pain described in the question make it less likely to be the primary involved structure in this case.
B) Abductor Pollicis Longus Tendon
Associated Muscle: Abductor Pollicis Longus
Nerve Innervation: Radial nerve (specifically, the posterior interosseous nerve)
Origin: Posterior surfaces of the ulna and radius, and the interosseous membrane
Insertion: Base of the first metacarpal and the trapezium
Relevance: Involved in thumb abduction and extension and can be involved in De Quervain's tenosynovitis, but the specific action causing pain here points more towards a primary extensor of the thumb.
C) Flexor Pollicis Longus Tendon
Associated Muscle: Flexor Pollicis Longus
Nerve Innervation: Median nerve (anterior interosseous branch)
Origin: Anterior surface of the radius and the adjacent interosseous membrane
Insertion: Base of the distal phalanx of the thumb
Relevance: This tendon flexes the thumb and is not typically involved in conditions associated with pain during thumb extension, making it less relevant to this case.
D) Extensor Pollicis Brevis Tendon
Associated Muscle: Extensor Pollicis Brevis
Nerve Innervation: Radial nerve (specifically, the posterior interosseous nerve)
Origin: Lower third of the posterior surface of the radius and the interosseous membrane
Insertion: Base of the proximal phalanx of the thumb
Relevance: This tendon, being a primary extensor of the thumb at the metacarpophalangeal joint and involved in the specific action that elicits pain in this case, is the most likely to be involved, especially considering its role in De Quervain's tenosynovitis.
E) Flexor Pollicis Brevis Tendon
Associated Muscle: Flexor Pollicis Brevis (has two heads: superficial and deep)
Nerve Innervation: Superficial head by the median nerve, deep head by the ulnar nerve
Origin: Superficial head from the flexor retinaculum and trapezium, deep head from the trapezoid and capitate bones
Insertion: Base of the proximal phalanx of the thumb
Relevance: This muscle flexes the thumb and is not involved in extending the thumb against resistance, making it less relevant to the patient's symptoms.
Considering the specific action that exacerbates the patient's pain (thumb extension against resistance with the wrist in a neutral position), the extensor pollicis brevis tendon is the most directly involved anatomical structure in the pathogenesis of this patient's condition. This aligns with the typical presentation of De Quervain's tenosynovitis, where the tendons in the first dorsal compartment of the wrist are affected.
FINAL ANSWER: D) Extensor Pollicis Brevis Tendon
