**Core Concept**
Heterotopic ossification (HO) is a condition characterized by the formation of bone tissue in soft tissues, often following trauma, surgery, or neurological disorders. In the context of HO around the knee joint, the underlying principle is the abnormal deposition of calcium and phosphate ions, leading to the formation of ectopic bone.

**Why the Correct Answer is Right**
The laboratory investigation of choice for diagnosing HO is serum alkaline phosphatase (ALP) levels. Elevated ALP levels indicate increased bone turnover, which is a hallmark of HO. This enzyme is essential for bone mineralization and is typically elevated in conditions characterized by bone formation. In HO, the increased ALP levels reflect the abnormal bone formation process.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum calcium levels may be normal in HO, as the condition primarily involves bone formation rather than calcium metabolism.
**Option B:** Serum creatinine kinase (CK) levels may be elevated in HO, but this is not the primary laboratory investigation of choice for diagnosing the condition.
**Option C:** X-rays are a valuable imaging modality for diagnosing HO, but they are not a laboratory investigation in the classical sense.

**Clinical Pearl / High-Yield Fact**
Elevated serum ALP levels in the context of HO can indicate a worse prognosis, as it suggests more aggressive bone formation.

**Correct Answer:** C. X-rays are a valuable imaging modality for diagnosing HO, but they are not a laboratory investigation in the classical sense.