Examining the interaction between content knowledge, inquiry proficiency, and writing proficiency is central to understanding the relative contribution of each proficiency on students' written communication about their science inquiry. Previous studies, however, have only analyzed one of these primary types of knowledge/proficiencies (i.e. content knowledge, inquiry proficiency, and writing proficiency) at a time. This study investigated the extent to which these proficiencies predicted students' written claims, evidence for their claims, and reasoning linking their claims to the evidence. Results showed that all three types of proficiencies significantly predicted students' claims, but only writing proficiency significantly predicted performance on evidence and reasoning statements. These findings indicate the challenges students face when constructing claim, evidence, and reasoning statements, and can inform scaffolding to support these challenges. [This paper was published in: "L@S '18: Proceedings of the Fifth Annual ACM Conference on Learning at Scale." ACM. (978-1-4503-5886-6).]