Which feature best distinguishes rabies virus from other enveloped RNA viruses like influenza and measles?

Explanation

## Distinguishing Rabies from Other Enveloped RNA Viruses ### Structural Hallmark of Rabies **Key Point:** Rabies virus is uniquely characterized by its **bullet-shaped (or cone-shaped) virion morphology**, which is pathognomonic and distinguishes it from all other enveloped RNA viruses. ### Comparative Virion Architecture | Feature | Rabies | Influenza | Measles | |---------|--------|-----------|----------| | **Virion Shape** | Bullet-shaped (pathognomonic) | Pleomorphic spherical | Pleomorphic spherical | | **Envelope Glycoprotein** | Single G protein | HA and NA spikes | H and F proteins | | **Genome** | Non-segmented negative-sense RNA | Segmented (8 segments) | Non-segmented negative-sense RNA | | **Replication Site** | Cytoplasm | Nucleus + cytoplasm | Cytoplasm | | **Electron Microscopy** | Distinctive bullet shape | Round with spike proteins | Round with spike proteins | ### Why Bullet Shape Matters **High-Yield:** The bullet-shaped morphology is visible on **electron microscopy** and is the gold-standard morphological identifier. This distinctive shape is due to the ribonucleoprotein core being wrapped asymmetrically within the envelope, creating the characteristic tapered appearance. **Clinical Pearl:** Rabies virions are often found in the **cytoplasm of neurons** and can be identified in brain tissue (especially hippocampus and medulla) using transmission electron microscopy during post-mortem diagnosis. ### Rabies Genome Organization **Key Point:** Rabies has a **non-segmented, negative-sense, single-stranded RNA genome** (~11.9 kb), encoding 5 structural proteins: - **N** (nucleoprotein) - **P** (phosphoprotein) - **M** (matrix protein) - **G** (glycoprotein — sole surface antigen) - **L** (RNA-dependent RNA polymerase) The single G protein is the only surface antigen and is responsible for viral attachment and immune recognition — this is why rabies vaccines target anti-G antibodies. ### Mnemonic for Rabies Identification **BULLET** = **B**asic morphology, **U**nique shape, **L**ong tapered end, **L**ow segmentation (none), **E**nvelope present, **T**wo-protein core [cite:Robbins 10e Ch 8] [cite:Harrison 21e Ch 189]