Final answer:
Hemoglobin levels do not decrease in polycythemia; instead, they increase due to the excess production of red blood cells. Conditions like anemia, severe hemorrhage, and renal disease decrease hemoglobin levels.
Step-by-step explanation:
The condition where hemoglobin levels do not decrease is D. Polycythemia. This is a blood disorder where there is an increase in the number of red blood cells (RBCs) and, consequently, an increase in hemoglobin levels.
Conditions that result in decreased hemoglobin levels include Anemia, Severe hemorrhage (which cause a loss of red blood cells and hemoglobin), and Renal disease (due to less production of erythropoietin which stimulates RBC production).