Upper Limb Anatomy, Human Anatomy, USMLE Step 1 - Full Vignette with Extended Explanations

Описание: A 65-year-old food service worker develops numbness and tingling in his right hand, specifically affecting the little finger and part of the ring finger, along with mild hand weakness and difficulty with fine motor tasks. Examination reveals selective sensory changes and motor findings, with normal thenar strength and negative provocative maneuvers. How would you approach localizing the lesion responsible for these symptoms based on the clinical and nerve conduction findings?

VIDEO INFO
Category: Upper Limb Anatomy, Human Anatomy, USMLE Step 1
Difficulty: Easy - Basic level - Suitable for medical students
Question Type: Differential Comprehensive
Case Type: Tricky Findings

Explore more ways to learn on this and other topics by going to https://endlessmedical.academy/auth?h...

QUESTION
A 65-year-old man who works in food service presents with numbness and tingling of the right hand for 2 months. He notices difficulty opening jars and fine motor tasks. The symptoms involve the little finger and the ulnar half of the ring finger on the palmar side. He denies neck pain, and the back of the hand feels normal. He reports an occasional morning cough after cooking shifts and returned from a vacation to Asia last year without illness....

OPTIONS
A. Ulnar nerve compression in Guyon canal zone 3 (superficial sensory branch) with palmar ulnar numbness and preserved dorsal ulnar hand sensation
B. Ulnar neuropathy at the cubital tunnel affecting both palmar and dorsal ulnar cutaneous territories with intrinsic hand weakness and positive elbow flexion-compression testing
C. C8-T1 radiculopathy from cervical spondylosis producing dermatomal sensory loss and thenar and hypothenar weakness
D. Median nerve entrapment in the carpal tunnel producing thumb, index, and middle finger paresthesias

CORRECT ANSWER
A. Ulnar nerve compression in Guyon canal zone 3 (superficial sensory branch) with palmar ulnar numbness and preserved dorsal ulnar hand sensation

EXPLANATION
Palmar numbness of the little finger and ulnar half of the ring finger with normal dorsal ulnar hand sensation and mild interosseous weakness localizes to the ulnar nerve at the wrist within Guyon canal. Preservation of dorsal ulnar cutaneous responses on nerve conduction confirms that the dorsal ulnar cutaneous branch, which arises proximal to the wrist, is intact. This pattern matches a zone 3 lesion (superficial sensory branch) if weakness is minimal, or a distal mixed lesion sparing the dorsal cutaneous branch; the stem highlights slowed sensory conduction to the superficial palmar branch with normal dorsal cutaneous studies-consistent with zone 3 sensory involvement.

Cubital tunnel lesions typically affect both palmar and dorsal ulnar cutaneous territories and often show positive elbow provocative testing. C8-T1 radiculopathy produces dermatomal loss and broader distal weakness, including thenar muscles, which are normal here. Median carpal tunnel neuropathy causes lateral palmar digit symptoms, not ulnar-side palmar numbness.

In summary, the correct answer is ulnar nerve compression in Guyon canal zone 3 with palmar ulnar numbness and preserved dorsal ulnar hand sensation.

Primary teaching point: The dorsal ulnar cutaneous branch arises proximal to the wrist; preserved dorsal sensation with palmar ulnar numbness localizes to Guyon canal.

Secondary teaching point: Shea & McClain s zones-zone 1 (mixed), zone 2 (motor), zone 3 (sensory)-help correlate exam and NCS findings.


Further reading:

Links to sources are provided for optional further reading only. The questions and explanations are independently authored and do not reproduce or adapt any specific third-party text or content.

---------------------------------------------------

Our cases and questions come from the https://EndlessMedical.Academy quiz engine - multi-model platform. Each question and explanation is forged by consensus between multiple top AI models (i.e. Open AI GPT, Claude, Grok, etc.), with automated web searches for the latest research and verified references. Calculations (e.g. eGFR, dosages) are checked via code execution to eliminate errors, and all references are reviewed by several AIs to minimize hallucinations.

Important note: This material is entirely AI-generated and has not been verified by human experts; despite stringent consensus checks, perfect accuracy cannot be guaranteed. Exercise caution - always corroborate the content with trusted references or qualified professionals, and never apply information from this content to patient care or clinical decisions without independent verification.

Clinicians already rely on AI and online tools - myself included - so treat this content as an additional focused aid, not a replacement for proper medical education. Visit https://endlessmedical.academy for more AI-supported resources and cases.

This material can not be treated as medical advice. May contain errors.

-------------------------------------...