**Core Concept**
The sternocleidomastoid (SCM) tumor, also known as fibromatosis colli, is a benign tumor of the SCM muscle, typically occurring in infants. It is characterized by a firm, painless mass in the SCM muscle, often associated with torticollis. This condition involves the growth of fibrous tissue within the muscle, leading to muscle shortening and deformity.

**Why the Correct Answer is Right**
Fibromatosis colli is a benign condition, often resolving spontaneously within the first few years of life. It is not a malignant tumor, and surgical intervention is usually reserved for cases with significant deformity or persistent symptoms. The SCM tumor is thought to result from abnormal muscle development and fibrosis, rather than a neoplastic process.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because SCM tumor is not typically associated with systemic symptoms or metastasis, unlike malignant tumors.
**Option B:** This option is incorrect because SCM tumor is not usually painful, unlike conditions such as SCM muscle strain or infection.
**Option C:** This option is incorrect because SCM tumor is not typically associated with neurological deficits, unlike conditions such as SCM muscle herniation or compression.

**Clinical Pearl / High-Yield Fact**
It is essential to distinguish SCM tumor from other causes of torticollis, such as congenital muscular torticollis, which may require physical therapy or surgical intervention. A thorough physical examination and clinical history are crucial in diagnosing and managing SCM tumor.

**Correct Answer: None.**