**Core Concept**
Primary CNS lymphoma is a rare and aggressive form of non-Hodgkin lymphoma that primarily affects the brain and spinal cord. These tumors can be difficult to diagnose due to their rarity and the fact that they often present with non-specific symptoms, making them a "ghost" in the diagnostic realm.

**Why the Correct Answer is Right**
Primary CNS lymphoma is often referred to as a "ghost cell tumor" because it can be challenging to detect on imaging studies, such as MRI or CT scans, until it has caused significant damage to the surrounding brain tissue. This is due to the fact that these tumors can infiltrate the brain parenchyma and cause a diffuse pattern of enhancement, making them difficult to distinguish from other types of brain lesions. The term "ghost cell tumor" highlights the elusive nature of this disease, which can lead to delayed diagnosis and poor outcomes if not treated promptly.

**Why Each Wrong Option is Incorrect**
**Option A:** Medulloblastoma is a type of malignant brain tumor that primarily affects children and is characterized by its rapid growth and tendency to spread to other parts of the central nervous system. Unlike primary CNS lymphoma, medulloblastoma is typically more visible on imaging studies and is often associated with symptoms such as hydrocephalus and cranial nerve deficits.

**Option B:** Acoustic neuroma is a type of benign tumor that affects the Schwann cells of the vestibulocochlear nerve and is characterized by its slow growth and tendency to cause hearing loss and balance problems. Unlike primary CNS lymphoma, acoustic neuroma is typically more visible on imaging studies and is often associated with symptoms such as tinnitus and vertigo.

**Option D:** Glioblastoma is a type of malignant brain tumor that arises from the astrocytes of the brain and is characterized by its rapid growth and tendency to spread to other parts of the central nervous system. Unlike primary CNS lymphoma, glioblastoma is typically more visible on imaging studies and is often associated with symptoms such as seizures and focal neurological deficits.

**Clinical Pearl / High-Yield Fact**
Primary CNS lymphoma is often associated with immunosuppression and can be a complication of diseases such as HIV/AIDS and organ transplantation. It is essential for clinicians to consider this diagnosis in patients with unexplained neurological symptoms and a history of immunosuppression.

**✓ Correct Answer: C. Primary CNS lymphoma**