The serum level of which of the following laboratory tests is decreased in both VDDR and VDRR?

In both Vitamin D Deficiency Rickets (VDDR) and Vitamin D Resistant Rickets (VDRR), the serum level of phosphate (Pi) is decreased. In these conditions, there is a fundamental issue with the metabolism of vitamin D which affects the absorption of phosphate in the intestines and its reabsorption in the kidneys.

Vitamin D plays a crucial role in the regulation of phosphate along with calcium; when vitamin D is deficient or resistant, the ability of the body to maintain normal levels of phosphate is impaired. This leads to lower serum phosphate levels, a common laboratory finding in both types of rickets.

While the levels of vitamin D, calcium, and parathyroid hormone can also be affected under these conditions, they do not consistently decrease in both VDDR and VDRR in the same manner that phosphate does. For instance, Vitamin D levels may be low in VDDR but can be normal or even elevated in VDRR due to the body’s resistance to its effects. Calcium levels can vary depending on the body's compensatory mechanisms and may not be reliably decreased in both conditions. Parathyroid hormone levels can be elevated due to the body's response to low calcium levels in VDDR but don't provide a consistent drop across both