Okay, let's tackle this question. The patient has loss of extension at the knee and decreased sensation on the anterior thigh. I need to figure out which nerve is damaged here.

First, I remember that the main nerves involved in the leg are the femoral nerve, sciatic, obturator, and others. The femoral nerve is responsible for the quadriceps muscle, which is crucial for knee extension. If the femoral nerve is damaged, the patient would have trouble extending the knee. The sensory area for the anterior thigh is also innervated by the femoral nerve, specifically the femoral cutaneous branches. So that fits.

Now, looking at the options. If the correct answer is femoral nerve, then the other options must be incorrect. Let's think about the other nerves. The sciatic nerve is more involved in the posterior thigh and lower leg, controlling muscles like the hamstrings. The obturator nerve is for hip adduction and medial thigh sensation. The saphenous nerve is a branch of the femoral but usually affects the medial leg, not the anterior thigh. So those other nerves wouldn't explain the loss of extension and anterior thigh sensation.

Clinical pearl: The femoral nerve injury can occur from trauma like hip fractures or compression from prolonged surgery in the supine position. Remember the key functions—quadriceps (knee extension) and anterior thigh sensation. That's a high-yield point for exams.

**Core Concept**
This question tests knowledge of femoral nerve anatomy and function. The femoral nerve innervates the quadriceps femoris muscle (responsible for knee extension) and provides sensory innervation to the anterior thigh via the femoral cutaneous branches.

**Why the Correct Answer is Right**
Damage to the **femoral nerve** results in weakness of knee extension due to loss of quadriceps function. Sensory deficits occur in the anterior thigh, as the femoral nerve supplies cutaneous branches to this region. Common causes include trauma (e.g., hip dislocation), compression (e.g., prolonged supine positioning), or iatrogenic injury during surgery. The knee reflex is also diminished in such cases.

**Why Each Wrong Option is Incorrect**
**Option A:** *Sciatic nerve* injury affects the posterior thigh, leg, and foot; it does not control knee extension.
**Option B:** *Obturator nerve* innervates hip adductors and medial thigh skin; anterior thigh sensation and knee extension are unaffected.
**Option C:** *Saphenous nerve* (a femoral branch) supplies medial leg and foot; anterior thigh sensation and knee extension are preserved.

**Clinical Pearl / High-Yield Fact**
Remember the **"3 Fs"** of femoral nerve function: **Flexion** of the hip, **Femoral** cutaneous sensory supply, and **Extension** of the knee via the quadriceps. This aids in localizing nerve injuries based on motor/sensory deficits.

**Correct Answer: D. Femoral nerve**

✓ Correct Answer: B. Femoral