What increases a patient's vulnerability to infections in agranulocytosis?

In agranulocytosis, there is a significant deficiency of granulocytes, which are a type of white blood cell key to the immune response. Granulocytes, particularly neutrophils, play a crucial role in the body's defense against infections, especially bacterial and fungal pathogens. When there is a reduction in these cells, the body's ability to mount an effective immune response is compromised, leading to an increased vulnerability to infections.

Because the core issue in agranulocytosis is the lack of these vital white blood cells, individuals are left with an insufficient capacity to fight off infections, making them more prone to illnesses. In this context, focusing on the deficiency of granulocytes directly correlates with the condition's risk factor for infections, thereby establishing why this answer is considered correct.

Other factors like low hemoglobin levels, excess platelets, or high neutrophil counts do not have the same direct impact on the increased risk of infection associated with agranulocytosis, as they do not pertain to the fundamental deficiency that defines the condition.