For a 7-year-old female showing signs of premature breast and hair development, which diagnostic step should be next?

In the case of a 7-year-old female exhibiting characteristics of premature breast development (thelarche) and other secondary sexual characteristics, the most clinically relevant diagnostic step is a brain MRI. This is because the presentation suggests a potential condition known as central precocious puberty, which is often due to early activation of the hypothalamic-pituitary-gonadal (HPG) axis. One potential etiology of central precocious puberty could be a central nervous system lesion, such as a tumor, that is stimulating the premature release of gonadotropins.

An MRI of the brain would help in visualizing any underlying abnormalities, such as a pituitary adenoma or hypothalamic tumor, that could be causing this early onset of puberty. Identifying such lesions is crucial for management, as treatment can vary significantly based on the underlying cause.

While assessment of thyroid function is important in the overall workup of precocious puberty, it is generally not the first-line test when signs of central precocious puberty are present. Bone age assessment is also useful to evaluate for advanced skeletal maturation which supports the diagnosis of precocious puberty but does not directly indicate any underlying structural abnormality. Lastly, a pelvic ultrasound is not typically indicated as an initial step in this