**Core Concept**
Paraplegia refers to a type of spinal cord injury that results in the loss of motor and sensory function below the level of injury, typically affecting the lower limbs. This condition is often caused by trauma, such as fractures or dislocations, which can damage the spinal cord and disrupt communication between the brain and the lower body.

**Why the Correct Answer is Right**
The correct answer, **B. Both the lower limb**, is accurate because paraplegia specifically involves the lower limbs, which are typically affected by injuries to the thoracic, lumbar, or sacral regions of the spinal cord. The spinal cord's damage disrupts the transmission of signals from the brain to the lower limbs, leading to weakness, paralysis, or loss of sensation in these areas. The upper limbs, which are controlled by the cervical region of the spinal cord, are usually spared in paraplegia.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because paraplegia specifically refers to the involvement of the lower limbs, not the upper limbs. Upper limb involvement is more characteristic of tetraplegia or quadriplegia.
**Option C:** Incorrect because paraplegia involves both lower limbs, not just one. The condition typically affects both lower limbs to some degree, depending on the level and severity of the spinal cord injury.
**Option D:** Incorrect because the correct answer is **B. Both the lower limb**, not any of the above. While paraplegia does involve the lower limbs, it does not typically involve the upper limbs or result in the loss of function in only one limb.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the level of spinal cord injury determines the extent of motor and sensory loss. A higher level of injury typically results in more severe impairment, while a lower level of injury may result in less severe impairment.