A middle aged man with back pain complains of pain and anaesthesia down the medial aspect of the lower leg. Which one of the following lower limb dermatomes corresponds best to the location of his symptoms?
a. L3
b. L4
c. L5
d. S1
e. S2

Answer. The answer is b. The skin of the medial lower leg is supplied by the L4 dermatome.  
Explanation. A dermatome is an area of skin supplied with afferent sensory fibres from a single spinal posterior nerve root. The dermatomes of the lower limb are derived from the lumbosacral plexus (L1 – S4), which has 3 main parts: one, the lumbar plexus (L1-L4 nerve roots with a contribution form T12 in around 50% of people); two, the lumbosacral trunk (L5 and usually including a branch from L4); and three, the sacral plexus (S1 – S4). In the limbs there is always some variation and overlap between dermatomes but in most the dermatomes of the lower limb are:
L1 – inguinal region
L2 – anterior and lateral upper thigh
L3 – anterior and medial lower thigh; front and sides of knee
L4 – medial ½ lower leg (front and back) below the knee
L5 – lateral aspect of the lower leg down over the anterior ankle and medial 2/3rds of the dorsum of the foot S1 – lateral ankle, lateral 1/3rd and sole of the foot
S2 – Posterior thigh and strip of skin over posterior calf; genitalia
S3 & S4 – perineal region
S5 perianal region (coccygeal plexus)  

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