What is the most likely diagnosis?
Acoustic schwannoma. With those characteristic MRI features, other differentials such as meningioma are far less likely.
MRI demonstrates a solid heterogeneous tumour at the left cerebellopontine angle, enlarging and partially occupying the left internal acoustic canal, exhibiting vivid contrast enhancement and mass effect over the adjacent cerebellar peduncle and pons. The fourth ventricle is partially effaced, however there is no obstructive hydrocephalus.