Obsessive-compulsive disorder (OCD) is a psychiatric illness characterized by excessive thoughts (obsessions) that lead to repetitive, maladaptive behaviors (compulsions). Common obsessions include a fear of contamination and/or a need to keep things orderly. Common compulsions include obsessive handwashing and counting in certain patterns.

OCD does not arise from a traumatic injury; it results from a combination of psychological and biological factors.

A compulsion is a behavior that one feels driven to perform repeatedly, aiming to reduce stress or prevent the occurrence of something horrible. These compulsive behaviors are quite often linked to obsessive thoughts in cases of Obsessive-Compulsive Disorder (OCD).

The two parts of Obsessive Compulsive Disorder (OCD) are the obsessions and the compulsions. Obsessions are the irrational, intrusive thoughts that distress the person with OCD, and compulsions are the actions that the person takes to try to alleviate the distress of the obsession. In Joanna's case, the fear of someone getting hurt is the obsession and the running up and down the stairs is the compulsion.

Obsessive-compulsive disorder (OCD) is an anxiety disorder characterized by obsessions (intrusive thououghts) and compulsions (intense needs to fulfill specific actions). A common obession is cleanliness, which can lend itself to compulsive hand washing or scrubbing.

Compulsions are repetitive behaviors that are undertaken to reduce anxiety. Some of the most common compulsions in those with OCD (obsessive compulsive disorder) are counting, handwashing, collecting, and organizing at a clinically problematic level. 

Unlike compulsions, which are behaviors that OCD patients feel they must do to prevent disaster or just to reduce their distress, obsessions are unwelcome, unhelpful thoughts that OCD patients actively attempt to ignore or suppress.

Given that Samantha must check for her keys a certain number of times until she is appeased, this begins to resemble ritualistic repetitive behavior. Repetitive ritualistic behavior is characteristic of obsessive-compulsive disorder. While checking for keys may be a part of everyday life, feeling the need to check for keys a specific and excessive number of times is not. 

Obsessions are recurring intrusive thoughts or worries that are characterized by the extreme difficulty patients have suppressing them. The obsession here is the patient's concern with her mother's health. Compulsions are repeated behaviors that a person feels compelled to perform in relation to a given obsession. In this case, the need to write in a ritualistic fashion as as to prevent a realization of the obsessive fear is the compulsive act.

OCD is a disorder characterized by obsessions (intrusive, distressing thoughts) and compulsions (actions taken to try to "fix" the intrusive thought). Common compulsions include excessive washing, turning items off, checking on loved ones, and arranging items. Flashbacks are a symptom of PTSD (Post Traumatic Stress Disorder) and are not associated with OCD.

All of the answers are examples of compulsive behaviors that result from anxiety, such as being "dirty" or leaving the door unlocked.