**Core Concept**
Rickets is a disease of bone development in children, characterized by softening of the bones due to impaired mineralization. Alkaline phosphatase is an enzyme involved in the mineralization process, and its levels can reflect the degree of bone turnover and mineralization.

**Why the Correct Answer is Right**
S. Alkaline phosphatase (ALP) is a persistant biochemical marker of rickets because it is elevated in response to increased bone turnover and osteoblastic activity. ALP is responsible for dephosphorylating phosphate groups, which is essential for the mineralization of bone matrix. In rickets, the body attempts to compensate for the lack of mineralization by increasing osteoblastic activity, leading to elevated ALP levels. This makes ALP a useful marker for monitoring the disease and its treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** S. Ca++ (Serum Calcium) levels may be normal or low in rickets, as the disease is characterized by impaired mineralization, not necessarily a decrease in calcium levels.

**Option C:** S. Acid phosphate is not a specific marker for rickets. While it may be elevated in some cases, it is not as sensitive or specific as ALP for diagnosing and monitoring rickets.

**Option D:** S. phosphate levels may be low in rickets due to impaired mineralization, but they are not a persistant marker of the disease. Phosphate levels can fluctuate and may not accurately reflect the underlying bone pathology.

**Clinical Pearl / High-Yield Fact**
It's essential to note that ALP is not specific to bone, as it is also elevated in other conditions such as liver disease and bone metastases. Therefore, a diagnosis of rickets should be based on a combination of clinical findings, radiographic evidence, and laboratory results, including ALP levels.

**✓ Correct Answer: B. S Alkaline phosphatase**