Wiki help me out with this practice exam question

[The Angel Of the South]

On my 150-question practice exam, I got through half the test before running into a problem that stumped me. (I had missed some; but with every wrong answer, I either understood the rationale afterward, or saw that my error was due purely to not reading carefully enough. That bodes well for my prospects of passing.) I have read the correct answer's rationale, but still don't really understand it. I hope someone can help me figure it out.

"[snip] Description of Procedure: The patient was brought to the OR and placed in the supine position. Preoperative antibiotics were given. He was placed in the supine position with all pressure points noted and well padded. He was prepped and draped in standard fashion. An incision was made approximately above the level of the cricoid. Blunt dissection was used to expose the anterior portion of the spine with carotid moved laterally and trachea and esophagus moved medially. I placed the needle into the disc spaces and at C5-C6. Distracting pins were placed in the body of C6. The disc was then completely removed at C5-C6. There was significant compression of the cord. This was carefully removed to avoid any type of compression on the cord. Multiple free fragments were noted. This was taken down to the level of ligamentum, and both foramen were opened. Part of the body of C5 was taken down to ensure that all fragments were removed and that heere was no additional constriction. There nerve root was then widely decompressed.

Machine bone allograft was placed into C5-C6, and a Zephyr plate was placed in the body C6 with a metal pin placed into the body at C5. Excellent purchase was obtained. Fluoroscopy showed good placement and meticulous hemostasis was obtained. Fascia was closed with 3-0 Vicryl, subcuticular 3-0 Dermabond for skin. The patient tolerated the procedure well and went to recovery in good condition."

a. 22554, 63081, 63082, 20931, 22845
b. 22551, 63081, 20931, 22840
c. 22551, 63081, 63082, 20931, 22845
d. 22554, 63081, 20931, 22840

(If anyone who wants to help doesn't have their codebook handy and would like me to post the code descriptions, let me know.) I thought the answer was C, but the answer key says A. I was confused by a number of things, mainly having to do with why I can't use 22551.

• Both codes say discectomy, and I don't really get the difference between "discectomy" in 22551 and "minimal discectomy" in 22554. What made this discectomy minimal?
• 22551 says disc space preparation; that's probably why I chose it. I thought the surgeon was working in the disc space, because the notes say "into the disc spaces." I guess I understand the difference between a disc space and an interspace: an interspace is not the disc space but between the disc spaces? (Or am I wrong and they mean the same thing?)
• 22554 says the minimal discectomy is to prepare the interspace other than for decompression. Well, there was decompression.