I have only seen three listings that classify the nursing diagnoses. (1) the list by NANDA classifying the diagnoses into Diagnostic Divisions. Knowledge Deficit is classified under the Teaching/Learning Diagnostic Division. (2) classification by Maslow's Hierarchy of Needs. Knowledge Deficit is classified under Safety and Security Needs. (3) classification by Gordon's Functional Health Patterns. Knowledge Deficit is classified under the Health Perception/Health Management Health Pattern.
Where do I find these lists, you ask? The NANDA list is in a book of care plans
by Doenges, Moorhouse and Murr in the beginning chapters. The Maslow and Gordon's listings are in Appendix A and Appendix B of Nursing Diagnosis Handbook: A Guide to Planning Care
, 7th Edition, by Betty J. Ackley and Gail B. Ladwig.
I have not seen a listing that classifies the nursing diagnoses as psychosocial. Are you sure this isn't something your nursing instructors might have given you at some time very early in your nursing lectures on the nursing process and care planning?