Final answer:
The next test in the evaluation of an obese female with oligomenorrhea and elevated testosterone level, suggestive of PCOS, would be to measure LH levels, as an elevated LH to FSH ratio is a key diagnostic marker of PCOS.
Step-by-step explanation:
A young obese female with oligomenorrhea and an elevated testosterone level is suggestive of polycystic ovary syndrome (PCOS). Given that her prolactin is within normal limits, the next step in the evaluation would typically involve assessing other hormones that play a critical role in the reproductive system and are often disrupted in PCOS. Among the options provided, LH (Luteinizing Hormone) is the most relevant to test next. This is because in PCOS, the LH to FSH ratio is often elevated, which is a key diagnostic criteria. LH, together with FSH (Follicle-Stimulating Hormone), regulates the menstrual cycle and plays a significant role in ovulation. An elevated LH level can lead to an increased production of testosterone and hence supports the diagnosis of PCOS.