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.

90% of people people with obsessive compulsive disorder (OCD) suffer from comorbid disorders. The most common comorbid disorders are social anxiety disorders. Of the 90% of OCD patients with comorbid disorders, 76% have social anxiety disorders. Approximately 63% suffer from mood disorders.

Obsessive compulsive disorder involves persistent, uncontrollable intrusions of unwanted thoughts (obsessions) and urges to engage in senseless rituals (compulsions). In this case, we know that Henry has compulsions, but we don't have evidence of obsessions. However, compared to the symptoms of panic disorder (recurrent panic attacks), post-traumatic stress disorder (anxiety and avoidance occurring after a traumatic event), and specific phobia (excessive fear over objects or situations that pose no real threat), we can conclude that the most likely diagnosis would be obsessive compulsive disorder.

People with histrionic personality disorder feel highly uncomfortable and/or offended when they are not the center of attention. They have rapidly shifting emotions, which are used to garner attention from others. 

Obsessive-compulsive disorder is an anxiety disorder, not a personality disorder.

The other listed conditions are all personality disorders.

The multi-axial system of the DSM (Diagnostic and Statistical Manual of Mental Disorders) was created in 1994 to address the "whole person" when making psychiatric diagnoses.

Axis I refers to the principal disorder in question (e.g., schizophrenia), axis II refers to personality disorders, axis III lists medical disorders that impact a client's psychiatric state, axis IV takes psychosocial factors into account (e.g., recent divorce leading to depression), and axis V gives an overall global functioning score on a scale of 0-100.