Final answer:
The Loa loa worm causes great pain when it crosses the blood-brain barrier and invades the central nervous system.
Step-by-step explanation:
The correct answer is b) Crosses the blood-brain barrier.
The Loa loa worm, also known as the African eye worm, is a parasitic nematode that causes loiasis, a disease endemic to West and Central Africa. When the worm infects humans, it can migrate through various tissues, including the skin, subcutaneous tissue, and even the conjunctiva of the eye. However, the most significant pain and complications are caused by the worm crossing the blood-brain barrier and invading the central nervous system. This can lead to severe symptoms such as migraines, encephalitis, and neurological disorders.