Final answer:
B. Obesity
The greatest risk factor for endometrial cancer in this 57-year-old postmenopausal patient is obesity because adipose tissue produces estrogens, leading to conditions that stimulate endometrial growth.
Step-by-step explanation:
The patient's greatest risk factor for developing endometrial cancer is her obesity. Obesity is associated with an increased risk of endometrial cancer due to higher levels of circulating estrogens in obese women.
Fat tissues can produce estrogens, leading to an estrogen-rich environment that can stimulate the growth of the endometrium. In this case, even though other factors such as nulliparity, postmenopausal status, use of hormone replacement therapy, and complex atypical hyperplasia are relevant, obesity stands out as the strongest risk factor in this scenario.
It should be noted that although complex atypical hyperplasia is a precancerous condition and increases the risk of progressing to endometrial cancer, it is not a risk factor in itself but rather a condition that develops as a consequence of exposure to risk factors, such as obesity in this instance.