Safety and Pharmacokinetics of Efinaconazole Topical Solution in Subjects With Mild to Severe Onychomycosis

Key Inclusion Criteria: * Male or female subjects of any race, 6 to 16 years of age (inclusive). PK Subset: Male or female subjects of any race, 12 to 16 years of age (inclusive). * Verbal and written informed consent/assent obtained from the subject and/or their parent or legal guardian. * Good general health, as assessed by the investigator, based on the subject's medical history, physical examination, and safety laboratory tests. * Target great toenail for all subjects, and both great toenails for subjects in the PK subset, must have evidence of toenail growth, per subject's report that monthly clipping is needed. * Subjects and their parents/legal guardians are willing to comply with study instructions and return to the investigational center for all required visits (a visit schedule with the length of each visit will be provided to ensure that the subject can meet the requirements and have adequate transportation). Key Exclusion Criteria: * Females who are pregnant, nursing an infant, or planning a pregnancy during the study period. * History of immunosuppression and/or clinical signs indicative of possible immunosuppression, as determined by the investigator, or known human immunodeficiency virus infection. * History of diabetes that is uncontrolled as determined by the investigator (diabetes that is controlled by diet or medication does not exclude a subject). * Presence of any toenail infection other than or in addition to dermatophytes, such as Scytalidium as determined by the investigator (candidal onychomycosis infection, concurrent with a positive dermatophyte culture, is acceptable).