**Core Concept**
The **modified radical mastectomy** is a surgical procedure for breast cancer that aims to remove the tumor while preserving some structures to reduce morbidity. This procedure is less invasive than a standard radical mastectomy. The **Scanlon’s modified radical mastectomy** is a variation of this procedure.

**Why the Correct Answer is Right**
In a **Scanlon’s modified radical mastectomy**, the goal is to balance oncological safety with the preservation of certain structures to maintain function and aesthetics. The **pectoralis minor muscle** is often preserved in this procedure, along with some axillary lymph nodes, to reduce the risk of complications such as lymphedema.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the pectoralis major muscle is sometimes partially removed or divided in modified radical mastectomies, depending on the tumor's location.
**Option B:** The sternocleidomastoid muscle is not directly involved in the field of a standard or modified radical mastectomy.
**Option C:** The rhomboid muscles are not typically considered in the surgical field for breast cancer surgeries like the modified radical mastectomy.

**Why Each Wrong Option is Incorrect (continued)**
**Option D:** This option is not applicable as the correct answer choice was not provided.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that preserving the **pectoralis minor muscle** and some axillary lymph nodes can help reduce the risk of **lymphedema**, a common complication of more radical breast cancer surgeries.

**Correct Answer:** D. Pectoralis minor muscle.