What are the minimum segments necessary to generate an ADT message?

MSH - Message header (Required)
EVN - Event type (Required)
PID - Patient identification (Required)
PV1 - Patient visit (Required)

What are the minimum segments necessary to generate an ORU message?

MSH - Message header (Required)
PID - Patient identification (Required)
PV1 - Patient visit (Required)
OBR - Observation request (Required)
OBX - Observation segment (Required, Repeatable)

What are the minimum segments necessary to generate an MDM message?

MSH - Message header (Required)
EVN - Event type (Required)
PID - Patient identification (Required)
PV1 - Patient visit (Required)
TXA - Document notification segment (Required)
OBX - Observation segment (Required, Repeatable)