A 1-year-old girl exhibits bowed legs and low growth rates. What is the most likely finding in her serum?

• A. Increased parathyroid hormone
• B. Normal 25-hydroxyvitamin D
• C. Increased 25-hydroxyvitamin D
• D. Normal parathyroid hormone

Answer: (B)

In the scenario presented, the 1-year-old girl shows signs consistent with rickets, a condition often caused by vitamin D deficiency. The key to understanding her serum findings lies in the role of vitamin D in calcium and phosphate metabolism, which are essential for normal bone development.

In this particular case, if the serum 25-hydroxyvitamin D level is normal, it suggests that there is no deficiency in vitamin D despite the presence of clinical symptoms such as bowed legs and low growth rates. This indicates that the bowing of the legs and growth issues could be due to other factors, such as a potential underlying genetic condition or malabsorption issues that may not directly relate to vitamin D levels.

Additionally, while parathyroid hormone levels are often elevated in vitamin D deficiency as the body tries to compensate for low calcium, here the normal 25-hydroxyvitamin D level suggests that the body is not responding to a deficiency. This is critical in ruling out conditions primarily related to vitamin D metabolism.

Thus, the most likely finding in her serum, in the context of her symptoms, would indeed be a normal level of 25-hydroxyvitamin D, supporting the diagnosis while excluding vitamin D deficiency as the etiology