Viral infections: Such as influenza, respiratory syncytial virus (RSV), or human immunodeficiency virus (HIV), which can cause neutropenia due to bone marrow suppression or immune system dysregulation. This is often seen in the context of a viral illness with systemic symptoms.

Medication-induced neutropenia: Certain medications like antibiotics (e.g., trimethoprim-sulfamethoxazole), anticonvulsants, and chemotherapy agents can cause neutropenia as a side effect. The history of recent medication use is crucial.
Autoimmune disorders: Conditions such as rheumatoid arthritis, lupus, or autoimmune neutropenia of infancy can lead to neutropenia due to immune system dysregulation and antibody-mediated destruction of neutrophils.
Bone marrow failure syndromes: Aplastic anemia, myelodysplastic syndromes, or leukemia can directly affect the production of neutrophils, leading to neutropenia.

Sepsis: Although not a cause of neutropenia per se, sepsis can initially present with neutropenia due to consumption of neutrophils in the setting of severe infection. Missing this diagnosis can be fatal.
Typhoid fever: Caused by Salmonella Typhi, this infection can lead to neutropenia and is critical to diagnose due to its severity and the need for specific antibiotic treatment.
Tuberculosis: Can cause neutropenia, especially in advanced cases or in immunocompromised patients. Given its potential severity and the need for specific treatment, it's crucial not to miss this diagnosis.

Congenital neutropenia: Rare genetic disorders that affect neutrophil production, such as Kostmann syndrome.
Cyclic neutropenia: A rare condition characterized by periodic episodes of neutropenia, often associated with other cytopenias.
Large granular lymphocytic leukemia: A rare form of leukemia that can cause neutropenia among other cytopenias.
Hypersplenism: Severe splenomegaly can lead to sequestration of neutrophils, resulting in neutropenia. This is more common in conditions like portal hypertension.