**Core Concept**
The patient's symptoms and examination findings suggest a diagnosis of Alzheimer's disease, a progressive neurodegenerative disorder characterized by the accumulation of amyloid beta plaques and tau protein tangles in the brain, leading to neuronal loss and cognitive decline.

**Why the Correct Answer is Right**
The Bielschowsky silver stain is a histopathological technique used to visualize neurofibrillary tangles, which are composed of tau protein. In Alzheimer's disease, the earliest change is the formation of neurofibrillary tangles in the hippocampus, a region critical for memory and learning. The hippocampus is one of the first areas affected in Alzheimer's disease, leading to the characteristic symptoms of memory loss and difficulty with spatial navigation.

**Why Each Wrong Option is Incorrect**
**Option A:** The cerebral cortex is affected later in the disease process, with widespread neuronal loss and synaptic degeneration.
**Option B:** The substantia nigra is primarily affected in Parkinson's disease, a different neurodegenerative disorder.
**Option C:** The basal ganglia are involved in movement disorders, such as Huntington's disease, and are not the primary site of early change in Alzheimer's disease.

**Clinical Pearl / High-Yield Fact**
The hippocampus is a key region for memory formation and retrieval, and its early involvement in Alzheimer's disease explains the characteristic memory loss and disorientation seen in patients with this condition.

**Correct Answer:** C.