This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
An infant with a history of delayed separation of umbilical cord now presents with recurrent pneumonia. The total count is 20,000/ml. Which of the following genetic defects is most likely present? 
Select and explain. 
A. Low levels of NADPH oxidase and negative DHR test, B. Mutation of Bruton tyrosine kinase gene, C. Excessive IgM with reduced IgG and IgA, D. Reduced phagocyte surface expression of Sialyl-Lewis x
D: Reduced phagocyte surface expression of Sialyl-Lewis x
Answer: D: Reduced phagocyte surface expression of Sialyl-Lewis x 
Explanation: This childs history of delayed separation of umbilical cord with recurrent pneumonia and peripheral leucocytosis is highly suggestive of leucocyte adhesion defect (LAD). Neutrophils have reduced adhesion, mobility and phagocytic ability. There are three types of LAD: LAD 1: Defective beta 2 integrin (CD18), leading to abnormal neutrophil chemotaxis and defective tight adhesion. LAD 2: Reduced phagocyte surface expression of Sialyl-Lewis x, leading to abnormal neutrophil rolling. LAD 3: Defective signaling for integrin activation, leading to abnormal neutrophil chemotaxis and adhesion. (Milder than type 1) Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition