Question:medium
A 19 year old boy fell from a motorbike onto his shoulder. The doctor diagnosed him as a case of Erb's paralysis. All of the following signs and symptoms will be observed except:
A 19 year old boy fell from a motorbike onto his shoulder. The doctor diagnosed him as a case of Erb's paralysis. All of the following signs and symptoms will be observed except:
Show Hint
Erb's palsy hits C5, C6; work out which movements those roots control, then find the one movement that is NOT one of them.
Updated On: Jul 8, 2026
- Loss of abduction at shoulder joint
- Loss of lateral rotation
- Loss of pronation at radioulnar joint
- Loss of flexion at elbow joint
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: Picture the mechanism of injury.
A fall directly onto the shoulder pushes the head away from the shoulder, stretching the upper roots of the brachial plexus, C5 and C6. This is the classic cause of Erb's palsy, and it fits the story of the motorbike fall.
Step 2: Match muscles to movements.
C5, C6 supply deltoid, supraspinatus, infraspinatus, teres minor, biceps, and brachialis. Losing these means the arm cannot abduct well, cannot rotate outward, and cannot flex fully at the elbow. The arm hangs limp and rotated in at the side, the well known "waiter's tip" posture.
Step 3: Focus on the forearm.
Biceps is also the strongest supinator. With it out of action, the forearm cannot supinate, and because nothing pulls it the other way, it tends to sit pronated. So supination is what is lost, not pronation, pronation still works and even becomes the resting position.
Step 4: Match this back to the options.
Loss of abduction, loss of lateral rotation, and loss of elbow flexion are all real findings in Erb's palsy. Loss of pronation is not, since pronation is spared.
Step 5: Conclude.
\[ \boxed{\text{Loss of pronation at radioulnar joint}} \]
A fall directly onto the shoulder pushes the head away from the shoulder, stretching the upper roots of the brachial plexus, C5 and C6. This is the classic cause of Erb's palsy, and it fits the story of the motorbike fall.
Step 2: Match muscles to movements.
C5, C6 supply deltoid, supraspinatus, infraspinatus, teres minor, biceps, and brachialis. Losing these means the arm cannot abduct well, cannot rotate outward, and cannot flex fully at the elbow. The arm hangs limp and rotated in at the side, the well known "waiter's tip" posture.
Step 3: Focus on the forearm.
Biceps is also the strongest supinator. With it out of action, the forearm cannot supinate, and because nothing pulls it the other way, it tends to sit pronated. So supination is what is lost, not pronation, pronation still works and even becomes the resting position.
Step 4: Match this back to the options.
Loss of abduction, loss of lateral rotation, and loss of elbow flexion are all real findings in Erb's palsy. Loss of pronation is not, since pronation is spared.
Step 5: Conclude.
\[ \boxed{\text{Loss of pronation at radioulnar joint}} \]
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