To solve this problem, we need to identify which of the given options is not a sexually transmitted disease (STD). Let's analyze each option:$\newline \bullet$Option 1: Syphilis$\newline$ - Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum.$\newline$ - It is transmitted through sexual contact.$\newline$ - Therefore, Syphilis is a sexually transmitted disease.$\newline \bullet$Option 2: Acquired Immuno Deficiency Syndrome (AIDS)$\newline$ - AIDS is caused by the Human Immunodeficiency Virus (HIV).$\newline$ - It is primarily transmitted through sexual contact, as well as through blood and from mother to child.$\newline$ - Therefore, AIDS is a sexually transmitted disease.$\newline \bullet$Option 3: Trichomoniasis$\newline$ - Trichomoniasis is a sexually transmitted infection caused by the parasite Trichomonas vaginalis.$\newline$ - It is transmitted through sexual contact.$\newline$ - Therefore, Trichomoniasis is a sexually transmitted disease.$\newline \bullet$Option 4: Encephalitis$\newline$ - Encephalitis is an inflammation of the brain, often caused by viral infections.$\newline$ - It is not typically transmitted through sexual contact.$\newline$ - Therefore, Encephalitis is not a sexually transmitted disease.$\newline$Based on the analysis, the correct answer is Option 4: Encephalitis, as it is not a sexually transmitted disease.