This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Leukocyte-adhesion deficiency 2 is due to the absence of __________. 
Select and explain. 
A. Selectin, B. LFA-1, C. MAC 1, D. Sialyl-Lewis X
D: Sialyl-Lewis X
Answer: D: Sialyl-Lewis X 
Explanation: Leukocyte  adhesion  deficiencies  (LADs)  stem  from  inherited defects in adhesion molecules that impair leukocyte recruitment to sites of infection, resulting in recurrent bacterial infections.

LAD1 is caused by defects in the β 2  chain that is shared by the integrins LFA-1 and Mac-1. 
LAD2 is caused by a defect in a fucosyl transferase that is required to synthesize functional sialyl-Lewis X, the ligand for E- and P-selectins.

​Reference-Robbins BASIC PATHOLOGY 10th edition pg-172