In a circle, an inscribed angle is half the measure of the arc that it subtends. In this case, the triangle LMN is inscribed in the circle with LM as the diameter, and angle L (at vertex L) measures 37°.
Since angle L is inscribed and subtends arc MN, we can calculate the measure of arc MN as follows:
\[ \text{Arc measure of } MN = 2 \times \text{inscribed angle at L} \]
Substituting the value we have:
\[ \text{Arc measure of } MN = 2 \times 37° = 74° \]
Therefore, the arc measure of MN is 74°.