Uworld specifically says that Psoas abscess means the patient will prefer flexion to avoid stretching the muscle. That's why Psoas did not make sense to me since the patient preferred extension, which would be stretching out the muscle.

On the other hand it's right on the vertebra and it's associated with TB.

The major motion of Psoas major is flexion of hip. This patient is held in "rigid extension", hence he does not want to flex. Psoas is the only muscle on this list that connects the lumbars to lower extremity "holds his right lower extremity"

Of all the options, psoas major is the only one that is really associated with the lumbar vertebrae.

Q. Lumborum involves the transverse process of L1 but Psoas Major originates from L1-L5