Final answer:
The next step is to measure TSH and prolactin levels to check for thyroid dysfunction and hyperprolactinemia, as they are common causes of amenorrhea in young women and other tests have ruled out pregnancy. The correct option is D.
Step-by-step explanation:
The most appropriate next step in the management of this patient, a 19-year-old college female who presents with amenorrhea and other nonspecific symptoms like minor weight gain and decreased energy, would be to evaluate her for possible thyroid and prolactin disorders.
Since a qualitative serum beta-hCG test has already ruled out pregnancy, the recommended tests are TSH and prolactin levels (Option D). This step is important because thyroid dysfunction and hyperprolactinemia are common causes of menstrual irregularities and amenorrhea in young women.
Furthermore, the absence of symptoms such as breast tenderness, abdominal fullness, nausea, and a normal BMI reduce the likelihood of a pregnancy-related issue or polycystic ovary syndrome (PCOS). The patient's symptoms are not suggestive of menopause, considering her age and menstrual history.