What early manifestation should a nurse observe for in a client suspected of having anorexia nervosa?

In cases of anorexia nervosa, amenorrhea, or the absence of menstrual periods, is often one of the earliest and most distinguishing signs observed in clients. This symptom occurs due to significant weight loss and extreme caloric restriction, which disrupt normal hormonal functioning and can lead to the cessation of menstruation.

The absence of menstruation is particularly notable in individuals of reproductive age, as it reflects the body's response to malnutrition and the energy deficit associated with the disorder. Amenorrhea is indicative of the physiological and psychological changes occurring within the body, which can include a drop in estrogen levels that is pivotal in regulating menstrual cycles.

The other manifestations listed, such as weight gain or frequent vomiting, are either not typical of anorexia nervosa or may be more associated with other eating disorders like bulimia nervosa. Intense physical activity can occur, but it often accompanies other symptoms rather than being an early manifestation. Thus, amenorrhea is a key indicator in the early detection of anorexia nervosa and can help guide appropriate interventions.