**Core Concept**
The "flowing wax" appearance on the anterior and posterior borders of vertebrae with a normal intervertebral disc space is indicative of a specific radiological finding associated with ligament calcification. This phenomenon is characterized by a characteristic pattern of calcification in the ligaments surrounding the spine.

**Why the Correct Answer is Right**
The "flowing wax" appearance is a classic radiological finding in diffuse idiopathic skeletal hyperostosis (DISH), a condition characterized by calcification and ossification of ligaments and tendons. The calcification of ligaments leads to a characteristic "flowing wax" or "candle wax" appearance on radiographs, particularly on the anterior and posterior borders of the vertebrae. The intervertebral disc spaces remain normal in size, which is a distinguishing feature of DISH from other conditions like ankylosing spondylitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because DISH is not typically associated with inflammatory back pain, which is a hallmark of ankylosing spondylitis.
**Option B:** This option is incorrect because osteoporosis is characterized by a decrease in bone density, not calcification of ligaments.
**Option C:** This option is incorrect because fibrous dysplasia is a condition characterized by the replacement of normal bone with fibrous tissue, not calcification of ligaments.

**Clinical Pearl / High-Yield Fact**
DISH is often associated with other conditions like hypertension, diabetes, and hyperparathyroidism, and patients may present with spinal stiffness, limited mobility, and pain.

**Correct Answer: C.**