**Core Concept**
Culex mosquitoes are a primary vector for various diseases, primarily caused by flaviviruses and filarial parasites. They transmit pathogens through their saliva while feeding on the blood of vertebrates.

**Why the Correct Answer is Right**
The correct answer is based on the understanding of the diseases spread by Culex mosquitoes. Culex mosquitoes primarily transmit diseases like West Nile virus (WNV), Japanese encephalitis (JE), and filariasis. Among the given options, filariasis is indeed a disease spread by Culex mosquitoes. However, the other diseases, such as dengue fever, chikungunya, and Zika virus, are primarily spread by Aedes mosquitoes, not Culex.

**Why Each Wrong Option is Incorrect**
* **Option A:** Dengue fever is spread by Aedes mosquitoes, not Culex. Aedes mosquitoes are the primary vectors for dengue, chikungunya, and Zika virus.
* **Option B:** Chikungunya is also spread by Aedes mosquitoes, not Culex. Aedes mosquitoes are the primary vectors for chikungunya, dengue fever, and Zika virus.
* **Option C:** Zika virus is spread by Aedes mosquitoes, not Culex. Aedes mosquitoes are the primary vectors for Zika virus, dengue fever, and chikungunya.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Culex mosquitoes primarily transmit flaviviruses like WNV and JE, while Aedes mosquitoes transmit dengue fever, chikungunya, and Zika virus.

**Correct Answer: D.**