"GABA" (Gamma-Amino Butyric Acid) is an inhibitory neurotransmitter, and is found throughout the neurons of the cortex. GABA deficiencies have been linked to anxiety disorders, and some anxiolytics (anxiety medications) work by increasing quantities of GABA available at neuron receptor sites.

The other choices are incorrect. "Serotonin" is primarily associated with regulating mood, appetite, and sleep, and is also implicated in nociception (sensation of pain). This neurotransmitter plays a major role in depression, but not necessarily anxiety. "Dopamine" is a major component of the reward system, and plays a role in arousal, motivation, motor control. "Norepinephrine" is mainly involved in preparing the body for action, for instance the 'flight or fight' response. It increases vigilance and alertness, focuses attention and improves memory recall and formation. Raised levels of norepinephrine can result in anxiety by way of increased stress. Last, "acetylcholine" is associated with motor neurons, and provides signals for the body to contract muscles. Drugs affecting this neurotransmitter thus may lead to paralysis or convulsions. In the brain acetylcholine plays a role in attention, arousal, and motivation.

"Anorexia nervosa" is an eating disorder characterized by a low weight, a fear of gaining weight, a strong desire to be thin, and food restriction. Although anorexia nervosa involves symptoms of anxiety (e.g. worried about gaining weight), it is not classified as an anxiety disorder. 

"Generalized anxiety disorder" (GAD) is an anxiety disorder characterized by chronic anxiety and excessive worry and tension, even when nothing seems to be wrong. Lucas' symptoms best fit a diagnosis of GAD. Since Lucas' anxiety does not only occur during a separation from his primary caregiver, he should not receive a diagnosis of separation anxiety disorder (SAD). All of the other answer choices do not accurately represent Lucas' symptomology.

 Anxiety disorders are marked by excessive anxiety such as apprehension, nervousness, or worry. Mood disorders have to do with severe changes in one's mood (e.g., depression). Psychotic disorders are characterized by losing touch with reality through hallucinations or delusions. Dissociative disorders involve losing contact with portions of their consciousness or memory.

Phobias are anxiety disorders that are characterized by a strong fearfulness of an object (e.g., snakes, spiders, cars) or situation (being in an elevator, being in small spaces) that presents no realistic danger. A panic attack is not a disorder.

The majority of eating disorders fall under the category “other specified feeding or eating disorders (OSFED). Previously, the percentage of cases was between 62.3 to 72.9%. With the current version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) the projected rate has gone down to 32.6 to 50.8%. It is difficult to get exact statistics for a variety of reasons, and this is also one of the reasons that many eating disorders are diagnosed in the "other" category.