**Core Concept**
The common peroneal nerve is a branch of the sciatic nerve that supplies the muscles of the lateral compartment of the leg, including the peroneus longus and peroneus brevis. It is a vital nerve for ankle and foot function, and its anatomical relationship with surrounding structures is crucial for its protection and preservation.

**Why the Correct Answer is Right**
The common peroneal nerve is closely related to the neck of the fibula, where it wraps around the lateral aspect of the bone. This anatomical relationship is due to the nerve's course around the fibula, which makes it susceptible to injury at this site, particularly in cases of fibular fracture or surgical procedures involving the lateral compartment of the leg. The close proximity of the nerve to the fibula's neck also makes it vulnerable to compression or stretching, which can lead to neurological symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** The shaft of the tibia is actually related to the tibial nerve, which is a separate branch of the sciatic nerve that supplies the muscles of the posterior compartment of the leg.

**Option C:** The lower tibio-fibular joint is not directly related to the common peroneal nerve, which is more closely associated with the fibula's neck.

**Option D:** The shaft of the fibula is not the correct site of the common peroneal nerve's relationship with the fibula, as the nerve is more closely associated with the fibula's neck.

**Clinical Pearl / High-Yield Fact**
The common peroneal nerve is often at risk during fibular fractures or surgical procedures involving the lateral compartment of the leg, particularly if the nerve is not properly identified and protected.

**✓ Correct Answer: B. Neck of fibula**