## **Core Concept**
The question tests the ability to identify which of the listed pathogens is not primarily transmitted through sexual contact. This requires knowledge of common sexually transmitted infections (STIs) and their typical modes of transmission.

## **Why the Correct Answer is Right**
The correct answer, **D. Rickettsia rickettsii**, is not primarily transmitted through sexual contact. *Rickettsia rickettsii* is the causative agent of Rocky Mountain spotted fever, a tick-borne disease. It is transmitted to humans through the bite of infected ticks, not through sexual contact.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Neisseria gonorrhoeae* is a well-known cause of gonorrhea, a common STI. It is primarily transmitted through sexual contact.
- **Option B:** *Chlamydia trachomatis* causes chlamydia, another prevalent STI. It is transmitted through sexual contact.
- **Option C:** *Treponema pallidum*, the causative agent of syphilis, is also primarily transmitted through sexual contact.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while many bacterial infections can be transmitted sexually, *Rickettsia rickettsii* stands out due to its unique transmission via tick bites. Understanding the primary modes of transmission for various pathogens is crucial for diagnosis and prevention.

## **Correct Answer:** D. Rickettsia rickettsii