Explanation: The correct answer is a. Complete blood count (CBC) with differential. A CBC with differential is a standard diagnostic test used to assess the levels of different blood cells, including white blood cells, which are crucial for the immune system. Abnormalities in the white blood cell count and differential can indicate potential immunodeficiency.

Incorrect choices:

b. A chest X-ray is not a specific diagnostic test for immunodeficiency disorders. It is typically used to assess the condition of the lungs and chest organs.

c. An electrocardiogram (ECG) is a test used to assess the electrical activity of the heart and is not related to the diagnosis of immunodeficiency disorders.

d. A urinalysis is a test used to assess the urine for various abnormalities and is not specific to diagnosing immunodeficiency disorders.