Deficient Knowledge diagnosis

I am doing a care plan for a man who recently had a PEG tube placed. I am required to do one psychosocial and one physiological diagnosis. For phys. I have "risk for falls r/t postoperative condition and nutritional deficit" First, does this sound like a winner? He has been noted as a fall risk, just dont know about r/t nutrition.

Most importantly...I have chosen Deficient knowledge to be my psych diagnosis on the client, but I don't want to build up a care plan without being sure that it is actually a psych, and not a phys (since I already have one). Any suggestions?

I did a care plan last week that my instructor didn't like, so I need to be sure I am distinushing between these 2 categories correctly. I used Disturbed Mental Process (caused by multi. CVA and UTI dementia) and Risk for Violence, Others-Directed (he had a past with hurting staff). She said both of them were psych, and I don't agree. I believe both could be either categorty, but I chose disturbed mental process for phys because it was causing, and was due to real psychological events. Anyone have any insight as to how I can distinguish between the "psychosocial and physiological" diagnoses to get through school? And this careplan?? I have tried searching the internet already with no real answer I can trust..