Anatomy and Physiology
These items test clinically applied structure and function: which nerve is injured with which fracture, what lies in a canal, and how a physiological variable is regulated.
Core rules
- Brachial plexus: roots C5–T1. A midshaft humeral fracture injures the radial nerve (wrist drop); a supracondylar fracture injures the median nerve; a medial epicondylar injury injures the ulnar nerve.
- Common peroneal nerve injury at the fibular neck causes foot drop.
- Inguinal canal: spermatic cord in men, round ligament in women; indirect hernia passes lateral to the inferior epigastric vessels through the deep ring.
- Cardiac output = heart rate × stroke volume; stroke volume rises with preload and contractility and falls with high afterload.
- Acid–base: anion gap = sodium − (chloride + bicarbonate); normal is 8–12 mmol/L.
Worked example
A man with a midshaft humeral fracture cannot extend his wrist or fingers and has numbness over the dorsum of the hand. The radial nerve runs in the spiral groove against the humeral shaft, so this is a radial nerve palsy. Check and document the nerve status before and after reduction.
Traps
- Attributing wrist drop to the ulnar nerve.
- Confusing direct and indirect inguinal hernia relations to the inferior epigastric vessels.
- Forgetting that a high anion gap metabolic acidosis suggests ketoacidosis, lactic acidosis or poisoning.
- Mixing up the cranial nerve supplying the lateral rectus (abducens) with the one supplying the superior oblique (trochlear).