**Question:** A pt. presents with defective adduction of the hip joint and pains in the hip and knee joint. Which nerve is involved?

**Core Concept:**
The movement of the hip joint is primarily controlled by the sciatic nerve, which is a large mixed sensory and motor nerve in the lower extremity. It originates from the L4, L5, S1, S2, and S3 spinal nerves and forms the lumbar plexus, which then splits into two branches: the common fibular nerve (peroneal nerve) and the common obturator nerve (obturator nerve). The obturator nerve innervates the adductor muscles of the thigh, which are responsible for adduction of the hip joint. The pain in the knee joint is likely due to referral pain, as the obturator nerve also supplies sensory innervation to the knee joint.

**Why the Correct Answer is Right:**
The correct answer, Option C (Obturator nerve), is right because the obturator nerve is responsible for innervating the adductor muscles in the thigh, which are involved in the movement of the hip joint, specifically adduction. The pain experienced by the patient in both the hip and knee joint is likely due to referred pain, as the obturator nerve not only innervates the muscles involved in hip adduction but also supplies sensory innervation to the knee joint.

**Why Each Wrong Option is Incorrect:**
A. The common fibular nerve (peroneal nerve) is part of the common fibular nerve complex and is responsible for innervating the muscles involved in dorsiflexion and abduction of the ankle and foot, not adduction of the hip joint.

B. The common obturator nerve (obturator nerve) is responsible for innervating the muscles involved in adduction of the hip joint, making it incorrect for this question.

D. The L5 nerve is part of the lumbar plexus, which includes the sciatic nerve that innervates the muscles involved in hip adduction. However, it is incorrect to single out L5 as the correct answer since the entire sciatic nerve is responsible for adduction of the hip joint.

**Clinical Pearl:**
In cases where patients present with referred pain and motor dysfunction, it is essential to consider the involvement of the obturator nerve, as it is responsible for adduction of the hip joint and innervates the knee joint for sensory function. This can help in narrowing down the differential diagnosis and guide further diagnostic and therapeutic interventions.