**Core Concept**
A sure sign of malignancy refers to a characteristic radiological feature or clinical presentation that strongly suggests the presence of cancer. In radiology, specific patterns or signs can indicate malignancy, helping clinicians to make informed decisions about patient care. These signs are crucial for early detection and diagnosis of cancer.

**Why the Correct Answer is Right**
The correct answer is **lytic bone lesions**. Lytic bone lesions are a type of bone destruction that is often associated with malignancy. They occur when cancer cells invade and destroy bone tissue, leading to radiolucent areas on radiographs. This is because cancer cells can stimulate osteoclasts, which break down bone, or inhibit osteoblasts, which build bone. Lytic lesions are a classic sign of metastatic bone disease, particularly from prostate, breast, or lung cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** **Sclerotic bone lesions** are often associated with Paget's disease, fibrous dysplasia, or osteoblastic metastases, which are not typical of malignancy.

**Option B:** **Bone fractures** can be caused by a variety of factors, including osteoporosis, trauma, or infection, and are not a specific sign of malignancy.

**Option C:** **Bone cysts** can be benign or malignant, but are not a sure sign of malignancy.

**Clinical Pearl / High-Yield Fact**
When evaluating radiographs, remember that lytic bone lesions are a red flag for malignancy, particularly in the context of a new or rapidly progressive lesion.

**Correct Answer: C. Lytic bone lesions**