**Core Concept**
Intercostobrachial nerve injury is a common complication after mastectomy, leading to sensory disturbances along the medial aspect of the arm. This nerve arises from the second and third intercostal nerves, providing sensory innervation to the medial arm.

**Why the Correct Answer is Right**
The intercostobrachial nerve is responsible for transmitting sensory information from the medial aspect of the arm, including the skin and subcutaneous tissue. Injury to this nerve during mastectomy can result in numbness, tingling, or pain along the medial aspect of the arm. This nerve is particularly vulnerable during mastectomy due to its proximity to the surgical site.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the long thoracic nerve, which innervates the serratus anterior muscle, is not responsible for sensory innervation of the arm.
**Option B:** This option is incorrect because the medial pectoral nerve primarily innervates the pectoralis major muscle and does not provide sensory innervation to the arm.
**Option C:** This option is incorrect because the medial antebrachial cutaneous nerve is responsible for sensory innervation of the medial forearm, not the medial aspect of the arm.

**Clinical Pearl / High-Yield Fact**
The intercostobrachial nerve is often sacrificed during mastectomy to improve surgical access and minimize the risk of recurrence. However, this can lead to sensory disturbances along the medial aspect of the arm, which may be a source of patient discomfort.

**Correct Answer:** A. Intercostobrachial nerve injury.