which of the following is an agent of a sexually transmitted disease?

Step 1: Analyze the question We need the organism that causes a sexually transmitted disease (STD/STI).

Step 2: Review each option

• A. Entamoeba histolytica: a protozoan that causes amoebic dysentery. It spreads through contaminated food and water (faecal-oral route), so it is not an STD.
• B. Salmonella typhi: a bacterium that causes typhoid fever. It also spreads through contaminated food and water, so it is not an STD.
• C. Treponema pallidum: a spirochete bacterium that causes syphilis, a classic STD. It spreads mainly through sexual contact, and also from mother to child during pregnancy.
• D. Clostridium tetani: a bacterium that causes tetanus. It enters the body through wounds contaminated with soil, so it is not an STD.

Step 3: Answer Only Treponema pallidum causes a sexually transmitted disease.

Quick tip: Learn the main STD pathogens in pairs:

• Syphilis → Treponema pallidum
• Gonorrhoea → Neisseria gonorrhoeae
• Chlamydia → Chlamydia trachomatis
• Trichomoniasis → Trichomonas vaginalis
• AIDS → HIV (a virus)

Also remember that Entamoeba and Salmonella are linked to food and water, and Clostridium tetani is linked to wounds.