**Core Concept**
Echinococcus is a parasitic tapeworm that primarily causes cystic echinococcosis (also known as hydatid disease) through the ingestion of contaminated feces or contact with infected animals. This condition is not typically considered a sexually transmitted infection (STI).

**Why the Correct Answer is Right**
Echinococcosis is primarily transmitted through the fecal-oral route, where the parasite's eggs are ingested, usually through contaminated food, water, or direct contact with infected animals. This route of transmission is distinct from the typical modes of STI transmission, which involve direct or indirect contact with an infected partner's bodily fluids, such as blood, semen, or vaginal secretions. The primary hosts of Echinococcus are canines, and the parasite's lifecycle does not involve human-to-human transmission through sexual contact.

**Why Each Wrong Option is Incorrect**
**Option A:** Candida albicans is a fungus that can cause vaginal yeast infections in women, which is considered an STI. However, it is not typically transmitted through sexual contact but rather through an imbalance of the vaginal microbiome.

**Option C:** Molluscum contagiosum is a viral skin infection caused by a poxvirus, which can be transmitted through skin-to-skin contact, including sexual contact. Therefore, it is considered an STI.

**Option D:** Group B streptococcus is a bacterium that can be transmitted from mother to child during childbirth or through sexual contact. While it is not typically considered an STI, it can be transmitted through sexual contact, making it a potential STI.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while Echinococcus is not an STI, other parasitic infections like Toxoplasma gondii can be transmitted through sexual contact with infected cats or contaminated feces.

**✓ Correct Answer: B. Echinococcus**