Thanks for writing in and sorry about the delay.
Main information given was proteinuria, high complement (c3) level, and low
back pain with morning stiffness. Any connection between the three?
One of several causes of proteinuria is
systemic lupus erythematosis which can present with arthritc
pain. This more often occurs in women. May present with fatigue, muscle aches, rash (eg butterfly rash on the face). The eyes, heart, lungs, brain, and musculoskeletal system may also be involved. Some labs like ANA and anti-CCP would help. I would suppose a low complement count to be more indicative of autoimmune disorder.
A high complement level with low
back pain and stiffness may be assosciated with an inflammatory condition. In a young man with low back morning stiffness, worse with inactivity, anklyosing spondilytis (AS) should be considered. Occurs in young men. This condition should improve with activity. Pelvic xray may help see inflamatory changes in the sacroiliac joints (sacroilitis). Probably want lumbar xray too. A late finding would be the "bamboo" looking spine. May also check sed rate and crp as inflamatory markers. If indeed this in AS, spinal extension exercise, walking and
NSAID are indicated.
Scoliosis (a lateral curvature of the spine) could cause
back pain. Depends on the severity of the curve. Hitting one's tailbone hurts initially but shouldn't cause this amount of
pain in the back.
Kidney problems can potentially cause low
back pain but probably are not connected with a high complement count unless something else (inflammatory) is going on.
All the best to you. Keep us posted.
