**Core Concept**
Patey's mastectomy is a type of modified radical mastectomy, which involves the removal of the breast, the underlying fascia of the pectoralis major muscle, the pectoralis minor muscle, and a level III axillary lymph node dissection. This surgical procedure aims to remove cancerous tissue while preserving the function of the pectoralis major muscle.

**Why the Correct Answer is Right**
The key step not performed in Patey's mastectomy is the en bloc removal of the pectoralis minor muscle, which is characteristic of the Halsted radical mastectomy. This omission helps to preserve the integrity and function of the pectoralis major muscle, reducing the risk of shoulder dysfunction and improving postoperative outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, but typically, it would be a distractor that is either unrelated to Patey's mastectomy or a step that is actually performed during the procedure.
**Option B:** This option might be a plausible distractor, but without the specific details, it's hard to pinpoint why it's incorrect.
**Option C:** This option could be a distractor that describes a step performed in a different type of mastectomy or a step that's not relevant to Patey's mastectomy.
**Option D:** This option might be a distractor that describes a step not performed in Patey's mastectomy, but without more context, it's hard to determine its accuracy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Patey's mastectomy is a variation of the radical mastectomy, and the key difference lies in the preservation of the pectoralis minor muscle, which is characteristic of the Halsted procedure.

**Correct Answer: D.**