Normal Lymph Node: The presence of a single lymph node with a central echogenic hilum in the right inguinal region, measuring 1.0 x 0.4 cm, is likely a normal finding. The central echogenic hilum is characteristic of a lymph node with a normal fatty hilum, and the size is within the range of normal lymph nodes.

Reactive Lymphadenopathy: This condition could be due to a variety of causes such as infection, inflammation, or immune response. The lymph node's size and appearance could be consistent with a reactive process.
Benign Lymph Node Hyperplasia: This is a condition where lymph nodes become enlarged due to an increase in the number of lymphocytes, but without any malignant transformation.

Lymphoma: Although less likely given the small size and solitary nature of the lymph node, lymphoma must be considered, especially if the patient has systemic symptoms or a history of lymphoma.
Metastatic Disease: A solitary lymph node in the inguinal region could potentially harbor metastatic disease from a primary malignancy, particularly from the lower extremities, genitalia, or anal region.

Castleman Disease: A rare lymphoproliferative disorder that can present with lymphadenopathy, which might be considered if the lymph node has atypical features or if the patient has systemic symptoms.
Kikuchi-Fujimoto Disease: A rare form of lymphadenitis that can mimic lymphoma or other malignant processes, typically presenting with cervical lymphadenopathy but could potentially involve inguinal lymph nodes.
Sarcoidosis: Although more commonly affecting the lungs and intrathoracic lymph nodes, sarcoidosis can involve any lymph node group, including the inguinal region, and would be a rare consideration for a solitary lymph node finding.