In a teenage girl with symptoms suggestive of fatigue, what is the most appropriate test to rule out vitamin D deficiency?

The most appropriate test to rule out vitamin D deficiency in a teenage girl presenting with symptoms suggestive of fatigue is the measurement of 25-hydroxyvitamin D. This is considered the primary form of vitamin D that circulates in the blood and is the best indicator of an individual's vitamin D status. It reflects both dietary intake and sunlight synthesis, providing a comprehensive picture of vitamin D levels.

In clinical practice, checking for 25-hydroxyvitamin D allows healthcare providers to determine if a deficiency exists, which can contribute to fatigue among other symptoms. Other forms of vitamin D, such as 1,25-dihydroxyvitamin D, are produced as a result of metabolism in the body and do not accurately indicate vitamin D stores; they are typically tested in conditions where there is a suspicion of metabolic disorders related to calcium and phosphorous metabolism. Cholecalciferol and ergocalciferol refer to forms of vitamin D (D3 and D2, respectively) and are not direct measures of vitamin D status in the body.

In summary, measuring 25-hydroxyvitamin D is essential for assessing and ruling out vitamin D deficiency, making it the most appropriate choice in this scenario.