Delusional Disorder: This is the most straightforward diagnosis when considering delusional syndrome, as it is characterized by the presence of one or more delusions for a month or longer. The delusions can be non-bizarre (i.e., within the realm of possibility) or bizarre, and the person's behavior and emotions are not markedly impaired apart from the direct impact of the delusions.

Schizophrenia: While schizophrenia involves a broader range of symptoms including hallucinations, disorganized speech, and negative symptoms, delusions are a key component. The presence of prominent delusions, especially if accompanied by hallucinations, could suggest schizophrenia.
Bipolar Disorder with Psychotic Features: During manic or depressive episodes, individuals with bipolar disorder can experience delusions, which would be considered psychotic features. The presence of mood symptoms alongside delusions could point towards this diagnosis.
Major Depressive Disorder with Psychotic Features: Similar to bipolar disorder, severe depression can sometimes include psychotic symptoms such as delusions. The key here would be the presence of depressive symptoms alongside the delusions.

Neurosyphilis: This is a potentially treatable condition that can cause delusions among other neurological and psychiatric symptoms. Missing this diagnosis could lead to severe and irreversible neurological damage.
Temporal Lobe Epilepsy: Certain types of epilepsy, particularly those involving the temporal lobe, can cause psychotic symptoms including delusions. Failing to diagnose and treat this condition could lead to ongoing seizures and worsening of symptoms.
Vitamin B12 Deficiency: A deficiency in vitamin B12 can lead to neurological and psychiatric symptoms, including delusions. This is a treatable condition, and missing it could result in permanent neurological damage if not addressed.

Charles Bonnet Syndrome: This condition involves visual hallucinations in individuals with visual impairment, but can sometimes include delusional components. It's relatively rare and often overlooked.
Cotard's Syndrome: A rare neuropsychiatric disorder in which a person believes they are dead or do not exist. This could be considered under the umbrella of delusional disorders but is distinct and rare.
Capgras Syndrome: A delusional misidentification syndrome where an individual believes a friend, spouse, parent, or other close family member has been replaced by an identical impostor. It's a rare condition that can be associated with schizophrenia or other neurological disorders.