Where is the proper placement of the central ray for a lateral projection of the 1st digit of the hand?

interphalangeal joint
metacarpophalangeal joint
distal interphalangeal joint
proximal interphalangeal joint

metacarpophalangeal joint

Rationale: The first digit, or thumb, is different than the other digits anatomically. It is comprised of a distal phalanx and a proximal phalanx. Therefore, its joints are the interphalangeal joint (IP) distally, the metacarpophalangeal joint (MCP), and the carpometacarpal joint (CMC) proximally. Due to the anatomical difference, the central ray is directed to the MCP joint to allow for proper collimation and inclusion of anatomy.