Final answer:
The first joint in index finger diagnosis for children is called the qi bar, a concept derived from traditional Chinese medicine, corresponding to the metacarpophalangeal joint.
Step-by-step explanation:
In index finger diagnosis in children, the first joint is referred to as the qi bar. This term comes from traditional Chinese medicine where different parts of the body, including the index finger, are believed to correlate with the health of various internal organs and systems. The metacarpophalangeal (knuckle) joint, which is the first joint of the finger from the hand, is one of the several joints in the body that can be palpated for diagnostic purposes.