Final answer:
Stephanie's sleep disturbances are likely due to a tumor in her thalamus (option d), which plays a key role in regulating consciousness, sleep, and alertness by filtering and relaying sensory information.
Step-by-step explanation:
Stephanie reports that she suddenly cannot sleep because she is unable to tune out noises such as the humming of the refrigerator or an occasional car driving down the street. After undergoing an MRI, she finds out that she has a tumor in her thalamus.
The thalamus is a major hub for information traveling between the spinal cord and cerebrum, filtering sensory information to the cerebrum and relaying sensory signals to the cerebral cortex and motor signals to the spinal cord. It is also crucial for the regulation of consciousness, sleep, and alertness. Since it helps regulate consciousness, arousal, and sleep states, any impairment like a tumor could lead to sleep disturbances like the ones Stephanie is experiencing.