**Core Concept**
The pituitary gland is a small endocrine gland located at the base of the brain, playing a crucial role in regulating various bodily functions through the secretion of several hormones. Its position, nestled in the sella turcica, is critical in understanding how tumors can impact surrounding structures.

**Why the Correct Answer is Right**
A pituitary tumor, particularly a macroadenoma, can grow and expand, compressing or displacing the nearby structures. In this scenario, the pituitary gland's proximity to the optic chiasm is key. As the tumor grows, it can exert pressure on the optic chiasm, leading to its obliteration or distortion. The optic chiasm is a critical structure where the optic nerves from each eye cross over, transmitting visual information from each eye to the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** This structure is not typically affected by a pituitary tumor. The cavernous sinus is a venous structure located near the pituitary gland, but it is not directly related to the pituitary gland's function or the effects of a pituitary tumor.
**Option B:** This structure is not the correct answer because it is not directly impacted by a pituitary tumor. The sella turcica is the bony structure that houses the pituitary gland, but it is not the structure that would be obliterated by a tumor.
**Option C:** This structure is not the correct answer because it is not directly related to the pituitary gland's function or the effects of a pituitary tumor. The optic nerve is a separate structure from the optic chiasm and is not typically affected by a pituitary tumor.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to remember that the pituitary gland's proximity to the optic chiasm means that a pituitary tumor can cause visual disturbances, such as bitemporal hemianopia, due to compression of the optic chiasm.

**Correct Answer:** D. The optic chiasm is the correct answer because it is the structure that may be obliterated by a pituitary tumor due to its proximity and the tumor's potential to compress or distort this critical structure.