What the hell is that? (Besides a good example of a throckmorton sign.) The object the arrow is pointing at was first discovered by doctors in 1974. It has been subsequently discovered by a few very surprised patients. That right there is a "pelvic digit."
It's not a growth, or a bone spur. Most doctors call this the "eleventh finger," because most digits have articulations, like the fingers on a hand. In this x-ray, it's sticking out of the pelvis, but they can be anywhere in the pelvic area, including floating in the abdominal wall.
In truth, it's probably more a rib than a finger. Scientists think that, during early development, the pelvis and the spine don't always come together on time, causing an extra rib to start forming in the empty space. Only 41 pelvic digits have ever been found, so it's a hard phenomenon to study.
In general, pelvic digits are found by accident. They're entirely asymptomatic, so doctors stumble across them when they x-ray a patient's pelvis for other reasons. Papers on the pelvic digit stress that it's a harmless anomaly, and urge doctors not to confuse it with more dangerous conditions, or risk surgery trying to remove it.
This particular digit was eventually removed, but only because it happened to give its owner trouble. Even as pronounced as it is, it didn't act up until he was in his late 20s. When the patient was 29, his hip began to get stiff and sore; the pain gradually increased until, when he was 35, he went to the doctor. At no point did he suspect he had an extra proto-rib inside him. A quick surgery removed the pelvic digit, but it lives on in this x-ray.