**Question:** True about prions is -

A. Prions cause diseases by only infecting nervous system cells
B. Prions are proteins that cause transmissible spongiform encephalopathies (TSEs)
C. Prion protein exists only in humans
D. Prion diseases are always fatal

**Correct Answer:**

**Core Concept:**
Prions are misfolded proteins that can cause transmissible spongiform encephalopathies (TSEs), such as Creutzfeldt-Jakob disease (CJD) and bovine spongiform encephalopathy (BSE). Prion proteins are notoriously resistant to degradation by normal cellular proteases, allowing them to accumulate and cause neuronal dysfunction and death.

**Why the Correct Answer is Right:**
Prions are unique because they can convert normal cellular prion proteins into their misfolded and infectious forms, leading to the development of TSEs. They are responsible for diseases like CJD, BSE, and scrapie in animals. The correct answer (B) highlights this infectious property and the cause of TSEs by prion proteins.

**Why Each Wrong Option is Incorrect:**
A) Prions infecting nervous system cells alone is incorrect because TSEs can involve multiple organs and systems, not just the nervous system.
C) Prion protein existence in humans alone is incorrect as prions also exist in other mammals.
D) Prion diseases being only fatal is incorrect, as some prion diseases have varying outcomes, such as chronic wasting disease in deer and elk.

**Clinical Pearl:**
Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases caused by misfolded proteins, or prions. Prions can have different clinical presentations, but they all share common pathological features like spongiform degeneration and neuronal loss. Prion diseases are important to understand for clinicians dealing with neurological disorders, as they can mimic other conditions, and knowledge of prion biology can aid in diagnosis and management strategies, such as identifying at-risk groups and implementing preventive measures.