A six-year-old boy is referred by this child study team for speech difficulty. On examination the boys friendly and cooperative. His speech is clear and uses simple sentences with a limited vocabulary. Most notably, he confuses verb tense consistently. Parent reports he met most of his developmental milestones except his speech was delayed. His clinical presentation is most consistent with which of the following? a. Reading disorder b. Writing disorder c. Stuttering disorder d. Expressive language disorder