What characterizes periodic limb movement disorder (PLMD)?

Periodic limb movement disorder (PLMD) is primarily characterized by involuntary leg movements occurring during sleep. These movements typically consist of repetitive flexing and extending of the legs. PLMD can lead to disrupted sleep due to these movements, but the hallmark of the disorder itself is the involuntary nature of the leg movements rather than the awakenings or other sleep disturbances that might accompany it. While frequent awakenings might be a consequence of the leg movements, and difficulty falling asleep can be associated with other sleep disorders, it is the involuntary leg movements during sleep that distinctly define PLMD.

The other options, such as consistent nightmares or frequent awakenings, may represent symptoms of different sleep disorders and are not specific to PLMD. The defining factor lies in the involuntary leg activities that occur during sleep, which aligns with the provided answer.