**Core Concept**
The rabies virus is a lyssavirus that causes progressive and fatal disease in mammals, including humans. It is a viral infection that affects the central nervous system (CNS), particularly the brain and spinal cord, by replicating and spreading through the neurons.

**Why the Correct Answer is Right**
The rabies virus travels along the axons of neurons at an estimated speed of 250 mm/day. This rapid spread of the virus is facilitated by the virus's ability to bind to the neural cell adhesion molecule (NCAM) on the surface of neurons, allowing it to move along the axon through a process called "fast axonal transport." This mechanism enables the virus to quickly infect and kill neurons, leading to the characteristic symptoms of rabies, such as hydrophobia, aggression, and eventual death. The speed of 250 mm/day is a critical factor in the rapid progression of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** 100 mm/day is too slow for the rapid progression of rabies, which is characterized by a short incubation period and rapid spread of the virus through the CNS.
**Option B:** 150 mm/day is still slower than the actual speed of the rabies virus, which is necessary for the virus to cause the severe and fatal symptoms associated with the disease.
**Option C:** 200 mm/day is closer to the correct answer, but still underestimates the speed at which the rabies virus travels along the axons of neurons.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the speed of the rabies virus is a critical factor in determining the prognosis and treatment options for patients. Early detection and prompt post-exposure prophylaxis (PEP) with rabies immunoglobulin and vaccination can prevent the development of clinical symptoms and reduce the risk of death.

**✓ Correct Answer: D. 250 mm/day**