**Core Concept**
Patey's mastectomy is a type of modified radical mastectomy, which involves the removal of the breast tissue, underlying muscles, and lymph nodes. This surgical procedure aims to remove the cancerous tissue while preserving as much of the surrounding tissue as possible. The key difference between Patey's mastectomy and a traditional radical mastectomy lies in the extent of muscle removal.

**Why the Correct Answer is Right**
The Pectoralis major muscle is not removed in Patey's mastectomy. Instead, the surgeon typically removes the pectoralis minor muscle along with the breast tissue and lymph nodes. This approach helps to minimize the disfigurement and preserve the patient's arm mobility. The pectoralis major muscle is crucial for shoulder stability and arm movement, and its preservation is essential for maintaining the patient's functional ability.

**Why Each Wrong Option is Incorrect**
**Option B:** The skin of the breast is indeed removed in Patey's mastectomy, as part of the surgical procedure to excise the cancerous tissue.
**Option C:** The pectoralis minor muscle is actually removed in Patey's mastectomy, not preserved. This muscle is located beneath the pectoralis major and is often sacrificed to facilitate the removal of the breast tissue and lymph nodes.
**Option D:** The ductular system of the breast, including the ducts and lobules, is removed in Patey's mastectomy. This is part of the surgical procedure to excise the cancerous tissue and prevent recurrence.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Patey's mastectomy is a modified radical mastectomy, which means that it preserves some of the surrounding tissue, but not all. This approach is critical in minimizing the disfigurement and preserving the patient's functional ability.

**✓ Correct Answer: A. Pectoralis major**