Final answer:
A physician would investigate immunoglobulin levels, specifically IgM, and assess the history of recurrent bacterial infections in patients suspected of having X-linked agammaglobulinemia, a condition characterized by the absence of B cells due to a defect in the BTK gene.
Step-by-step explanation:
In a patient with X-linked agammaglobulinemia (XLA), a physician would look for signs of recurrent bacterial infections, particularly by extracellular pathogens, due to a significant reduction or absence of B cells and immunoglobulins resulting from a mutation in the BTK gene. This mutation disrupts the maturation of B cells, halting their differentiation at the pre-B-cell stage and preventing them from producing immunoglobulin M (IgM) and other antibodies required for fighting off infections.
Diagnostic tests would likely include measuring serum immunoglobulin levels, which would be very low or absent, and specifically checking for IgM antibodies using assays like agglutination tests. Since XLA is a genetic disorder, it often presents with severe infections early in life which could be deadly without treatment. Considering XLA is X-linked, it predominantly affects males who have only one X chromosome.