Which of the following is not a sexually transmitted disease?

Options

A. Syphilis
B. AIDS
C. Trichomoniasis
D. Encephalitis

Explanation

To determine which of the following is not a sexually transmitted disease (STD), we will analyze each option provided.

### Step-by-Step Solution:

1. **Understand what STDs are**: STDs are infectious diseases that are primarily transmitted through sexual contact. They can be caused by various microorganisms including bacteria, viruses, and protozoa.

2. **Evaluate the first option - Syphilis**:
- Syphilis is caused by the bacterium *Treponema pallidum*.
- It is transmitted through sexual contact.
- Therefore, syphilis is a sexually transmitted disease.

3. **Evaluate the second option - AIDS**:
- AIDS (Acquired Immunodeficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV).
- It is also transmitted through sexual contact.
- Thus, AIDS is a sexually transmitted disease.

4. **Evaluate the third option - Trichomoniasis**:
- Trichomoniasis is caused by the protozoan *Trichomonas vaginalis*.
- It is transmitted through sexual contact.
- Hence, trichomoniasis is a sexually transmitted disease.

5. **Evaluate the fourth option - Encephalitis**:
- Encephalitis is an inflammation of the brain, often caused by viral infections.
- It is typically transmitted by vectors such as mosquitoes (e.g., Culex mosquitoes).
- Encephalitis is not transmitted through sexual contact, making it not a sexually transmitted disease.

### Conclusion:
The option that is not a sexually transmitted disease is **Encephalitis**.

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