**Core Concept**
Candida albicans, Molluscum contagiosum, and Group B streptococcus are pathogens that can be transmitted through sexual contact, often associated with compromised immune systems or high-risk behaviors. In contrast, Echinococcus is a zoonotic parasite primarily transmitted through contact with contaminated feces or undercooked meat.

**Why the Correct Answer is Right**
Echinococcus, specifically Echinococcus granulosus, is the causative agent of cystic echinococcosis (hydatid disease). This parasite has a complex lifecycle involving canines as definitive hosts and herbivores as intermediate hosts. Humans are incidental hosts, typically acquiring the infection through accidental ingestion of contaminated feces or undercooked meat, rather than sexual transmission. The parasite's lifecycle and transmission dynamics are distinct from those of the other listed pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** Candida albicans is a commensal yeast that can become pathogenic in immunocompromised individuals and is often transmitted through sexual contact, particularly among men who have sex with men.

**Option C:** Molluscum contagiosum is a viral skin infection caused by the poxvirus, often spread through skin-to-skin contact, including sexual contact.

**Option D:** Group B streptococcus (Streptococcus agalactiae) can be transmitted through sexual contact, particularly among pregnant women and individuals with compromised immune systems.

**Clinical Pearl / High-Yield Fact**
The zoonotic transmission of Echinococcus highlights the importance of proper food handling and hygiene practices, particularly in regions where the parasite is endemic.

**✓ Correct Answer: B. Echinococcus**