**Core Concept**
The somatosensory area I, also known as the primary somatosensory cortex, is responsible for processing basic sensory information from the body, including touch, pressure, temperature, and proprioception. It is organized in a somatotopic manner, meaning that different parts of the body are represented by specific areas of the cortex. The somatosensory area I receives input from the dorsal column-medial lemniscus pathway and is crucial for the perception of tactile stimuli.

**Why the Correct Answer is Right**
The correct answer is A. Arm because the somatosensory area I has a large representation of the arm due to its high density of sensory receptors and its importance for fine motor movements. The arm is also responsible for a wide range of activities, from grasping and manipulating objects to performing complex movements, which requires a large amount of sensory information to be processed. The large representation of the arm in the somatosensory area I allows for precise and accurate processing of this information.

**Why Each Wrong Option is Incorrect**
**Option B:** The leg has a significant representation in the somatosensory area I, but it is not the largest. The leg is also responsible for weight-bearing and locomotion, but its representation in the somatosensory area I is not as extensive as that of the arm.
**Option C:** The back has a relatively small representation in the somatosensory area I, as it is primarily responsible for proprioception and is not as sensitive to tactile stimuli as other areas of the body.
**Option D:** The head has a relatively large representation in the somatosensory area I, particularly in the face and mouth areas, which are responsible for processing sensory information from the eyes, ears, and tongue. However, it is not the largest representation.

**Clinical Pearl / High-Yield Fact**
A classic exam question is to remember that the somatosensory area I is organized in a somatotopic manner, with the face and hands being represented in the most anterior and lateral parts of the cortex, and the legs and back being represented in the most posterior and medial parts.

**✓ Correct Answer: A. Arm**