This is because antibiotics act synergistically with the immune system to subdue infection, and it is critical for success that the host defenses are adequate.

Choice A is wrong because Age is wrong because age is a general factor that must be considered when choosing any medication, not just antimicrobials.

Choice C is wrong because Genetic heritage is wrong because genetic heritage is also a general factor that may affect the metabolism and excretion of medications, not the selection of antimicrobials.

Choice D is wrong because Previous medication reactions is wrong because previous medication reactions are also a general factor that may indicate allergies or intolerances to certain medications, not the selection of antimicrobials.

Normal ranges for immune function vary depending on the type and level of immunity measured, such as innate, adaptive, humoral or cellular immunity. Some common tests for immune function include complete blood count (CBC), immunoglobulin levels, complement levels, antibody titers and skin tests.

Normal ranges for these tests depend on the laboratory and the method used, but some examples are:

• CBC: white blood cell count 4.5-11 x 10^9/L; neutrophils 40-75%; lymphocytes 20-50%; monocytes 2-10%; eosinophils 1-6%; basophils 0-2%.

• Immunoglobulin levels: IgG 7-16 g/L; IgA 0.7-4 g/L; IgM 0.4-2.3 g/L; IgE 0-0.4 g/L.

• Complement levels: C3 0.9-1.8 g