**Core Concept**
The underlying principle being tested is the identification of heterotopic calcification, which refers to the abnormal deposition of calcium salts in tissues, and its potential causes. This condition can be associated with various diseases, including those affecting **parathyroid hormone** regulation and **vitamin D** metabolism.

**Why the Correct Answer is Right**
Given the presence of heterotopic calcification around bilateral knee joints, the next step would involve investigating the cause of this abnormal calcification. A key consideration is **hyperparathyroidism**, which can lead to increased calcium levels in the blood, potentially causing calcification in soft tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not directly relate to the common causes of heterotopic calcification.
**Option B:** Similarly, this option does not address the primary concern of identifying the underlying cause of the calcification.
**Option C:** This choice is also incorrect as it is not the most appropriate next step in diagnosing the cause of heterotopic calcification.

**Clinical Pearl / High-Yield Fact**
A crucial point to remember is that heterotopic calcification can be a sign of underlying metabolic disorders, such as **hyperparathyroidism** or **renal failure**, which necessitate further investigation.

**Correct Answer:** D. Serum calcium and phosphate levels, and parathyroid hormone assay, to investigate for hyperparathyroidism or other metabolic causes.