**Core Concept**
Organic psychosis refers to a group of psychiatric disorders caused by a medical condition or substance that affects the brain, leading to disturbances in thought, perception, and behavior. This condition is characterized by a loss of contact with reality, which is often reversible with treatment of the underlying medical condition.

**Why the Correct Answer is Right**
The characteristic symptom of organic psychosis is hallucinations. Hallucinations are sensory experiences that occur in the absence of external stimuli, and they are often auditory in nature. In organic psychosis, hallucinations are typically a result of a disruption in the normal functioning of the brain, often due to a medical condition or substance use. For example, in cases of neurodegenerative diseases such as Alzheimer's disease, hallucinations may occur due to the degeneration of brain cells in areas responsible for processing sensory information.

**Why Each Wrong Option is Incorrect**
**Option A:** Disorganized thinking is a symptom of schizophrenia, a type of functional psychosis, rather than organic psychosis.
**Option B:** Psychotic depression is a type of mood disorder that can feature psychotic symptoms, but it is not a characteristic symptom of organic psychosis.
**Option C:** Catatonia is a motor symptom that can occur in various psychiatric conditions, including schizophrenia, but it is not a characteristic symptom of organic psychosis.

**Clinical Pearl / High-Yield Fact**
It's essential for clinicians to consider underlying medical conditions when evaluating patients with psychotic symptoms, as treatment of the underlying condition may lead to resolution of the psychosis.

**Correct Answer: A. Hallucinations**