A Study of Pembrolizumab in Japanese Pediatric Participants With Solid Tumors or Lymphomas and Japanese Adult Participants With Merkel Cell Carcinoma (MK-3475-G21/KEYNOTE-G21)

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: Arm 1: * For participants with relapsed or refractory classical Hodgkin lymphoma (cHL) or primary mediastinal large B-cell lymphoma (PMBCL) * Has a confirmed diagnosis of relapsed or refractory cHL or PMBCL after the most recent therapy * Has radiographically measurable disease per Lugano classification * For participants with completely resected melanoma: * Has surgically completely resected and histologically/pathologically confirmed diagnosis of Stage IIB, IIC, III or IV cutaneous melanoma * Has not received any prior systemic therapy for their melanoma beyond surgical resection * All suspicious lesions amenable to biopsy are confirmed negative for malignancy * For participants with locally advanced or metastatic melanoma: * Has histologically confirmed diagnosis of locally advanced (unresectable Stage III) or metastatic (Stage IV) melanoma (including acral) not amenable to local therapy * Has radiographically measurable lesion(s) as defined by RECIST 1.1 * For participants with microsatellite instability-high (MSI-H)/mismatch repair deficiency (dMMR) solid tumors: * Has histologically/cytologically documented, locally-advanced, or metastatic solid malignancy that is incurable and has either (a) failed prior standard therapy, (b) for which no standard therapy exists, or (c) standard therapy is not considered appropriate by the participant and treating physic…