A patient is admitted to a psychiatric hospital after having been picked up by the police for making inappropriate sexual advances. A detailed psychiatric interview demonstrates deficits in memory, insight, judgement, personal appearance, and social behavior. The patient is witnessed experiencing a possible epileptic seizure. Over a period of several years, motor findings also develop, including relaxed, but expressionless facies, tremor, dysahria, and pupillary abnormalities. Which of the following tests performed on his cerebrospinal fluid would most likely be diagnostic?

**Core Concept**
The patient's symptoms suggest a neurodegenerative disorder with both psychiatric and motor manifestations, likely associated with a specific type of dementia. The combination of memory deficits, inappropriate behavior, and motor findings such as tremor and dysarthria points towards a diagnosis of a prion disease, specifically Creutzfeldt-Jakob disease (CJD).

**Why the Correct Answer is Right**
Creutzfeldt-Jakob disease is a rare, degenerative, fatal brain disorder caused by an abnormal form of a protein called a prion. The disease is characterized by rapid cognitive decline, personality changes, and motor dysfunction. The presence of a possible epileptic seizure and the development of motor findings such as tremor and dysarthria are consistent with this diagnosis. The diagnosis of CJD is often confirmed by the presence of 14-3-3 protein in the cerebrospinal fluid, which is a marker of neuronal injury.

**Why Each Wrong Option is Incorrect**
**Option A:** Glial fibrillary acidic protein (GFAP) is an intermediate filament protein that is expressed in astrocytes. While it can be detected in the cerebrospinal fluid in certain conditions, it is not a specific marker for Creutzfeldt-Jakob disease.

**Option B:** Neurofilament light chain (NfL) is a marker of axonal injury and can be elevated in various neurodegenerative diseases, including Alzheimer's disease and amyotrophic lateral sclerosis. However, it is not specific for CJD.

**Option C:** Tau protein is a microtubule-associated protein that is involved in neurofibrillary tangle formation in Alzheimer's disease and other tauopathies. While it can be detected in the cerebrospinal fluid in certain conditions, it is not a specific marker for CJD.

**Clinical Pearl / High-Yield Fact**
The 14-3-3 protein is a marker of neuronal injury and is often elevated in the cerebrospinal fluid of patients with Creutzfeldt-Jakob disease. This protein is involved in various cellular processes, including signal transduction and protein degradation.

**Correct Answer:** . **Correct Answer:** D. 14-3-3 protein.

✓ Correct Answer: B. FTA-ABS