Step 1: The hallmark of paranoid personality disorder is a pervasive and unjustified mistrust and suspiciousness of others, with a tendency to interpret the motives and actions of others as malevolent or hostile.
Step 2: Typical features include the belief that others are exploiting or deceiving the person, that friends are untrustworthy or disloyal, doubts about a partner's fidelity, and reading hidden threatening meaning into neutral or friendly remarks. Many also carry a sense of self-importance.
Step 3: Why the others are wrong: schizoid personality disorder is marked by social detachment and a restricted range of emotions, not suspiciousness. Schizotypal personality disorder shows odd beliefs, magical thinking and eccentric behaviour. Anankastic (obsessive-compulsive) personality disorder is dominated by perfectionism, orderliness and rigid control. None of these has suspiciousness as the core feature.
Step 4: Therefore suspiciousness points to paranoid personality disorder.