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: Skin
Ecthyma gangrenosum is caused by: 
Select and explain. 
A. Fungal infection, B. Pseudomonas aeruginosa, C. Staphylococcus, D. Streptococcus
B: Pseudomonas aeruginosa
Answer: B: Pseudomonas aeruginosa 
Explanation: Ecthyma gangrenosum is a localized cellulitis with necrotizing and ulcerative evolution occurring in immunocompromised or neutropenic patients, mainly caused by Pseudomonas aeruginosa. Two pathogenic mechanisms are recognized; one occurring as the result of the presence of P. aeruginosa during bacteriemia, and the second as a primary lesion. Treatment is systemic antibiotics. Ref: Dermatology by jean bolognia 2nd e p. 1093 ; Life-Threatening Dermatoses and Emergencies in Dermatology edited by Jean Revuz, Jean-Claude Roujeau, Francisco Kerdel, Laurence Valeyrie-Allanore, 2009, Page 126