Final answer:
The left atrial dimension of 68 mm obtained by M-mode echocardiography is best described as moderately dilated.
Step-by-step explanation:
The left atrial dimension of 68 mm obtained by M-mode echocardiography is best described as moderately dilated. In a normal heart, the left atrial dimension is generally smaller, typically around 30-40 mm. When it exceeds 40 mm, it is considered to be dilated. Mild dilation is classified as a dimension between 41-50 mm, moderate dilation between 51-60 mm, and severe dilation when it exceeds 60 mm. Therefore, a measurement of 68 mm falls within the moderately dilated range.