**Core Concept**
Osteomyelitis is a bone infection, typically caused by bacteria, that can lead to various radiological changes on X-ray. The earliest changes are crucial for early diagnosis and treatment.

**Why the Correct Answer is Right**
The earliest X-ray finding in osteomyelitis is the **periosteal reaction**, which occurs due to the body's inflammatory response to the infection. This reaction involves the periosteum, a layer of connective tissue that surrounds the bone, and can lead to the formation of new bone (bone formation) or the destruction of existing bone (bone resorption). The periosteal reaction is often visible on X-ray as a thickening of the periosteal layer or a new bone formation on the surface of the infected bone.

**Why Each Wrong Option is Incorrect**
* **Option A:** Soft tissue swelling is a common finding in osteomyelitis, but it is not the earliest X-ray finding. Soft tissue swelling can be seen on X-ray, but it is not as specific as the periosteal reaction for diagnosing osteomyelitis.
* **Option B:** Bone destruction or lytic lesions are characteristic of osteomyelitis, but they are not the earliest X-ray finding. Bone destruction occurs later in the disease process, after the periosteal reaction has begun.
* **Option C:** Joint space narrowing is not a typical finding in osteomyelitis. Joint space narrowing is more commonly associated with other conditions, such as osteoarthritis or rheumatoid arthritis.

**Clinical Pearl / High-Yield Fact**
The earliest X-ray finding in osteomyelitis is often subtle and may be missed on initial X-rays. Repeat X-rays and other imaging modalities, such as MRI or CT scans, may be necessary to confirm the diagnosis.

**Correct Answer:** C.