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: Medicine
Patients with underlying serious hea disease need to have their hemoglobin level kept above: 
Select and explain. 
A. 8 g/dl, B. 9 g/dl, C. 10 g/dl, D. 11 g/dl
D: 11 g/dl
Answer: D: 11 g/dl 
Explanation: Patients without serious underlying cardiovascular or pulmonary disease can tolerate hemoglobin levels above 8 g/dL; Patients with serious underlying cardiovascular or pulmonary disease need to have their hemoglobin levels kept above 11 g/dL. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 851