An 8-month-old is admitted to the hospital for the evaluation of possible middle ear infection and chronic diarrhea. Since birth, he has episodes of frequent loose stools. He has had 2 episodes of lower respiratory tract infection with bronchospasm, treated with intravenous antibiotics and bronchodilators. His parents are not related, and they report that their firstborn died of pneumonia at the age of 8 months. On examination, the patient appears irritable, screams, shakes his head, and tries to rub the right ear. Signs of dehydration are noted. His weight is < 5th percentile; his height is 5th percentile, and his head circumference is 50th percentile. His temperature is 38 C. Mucocutaneous changes suggestive of a fungal infection are noted. Right tympanic membrane is erythematous and bulging, with poor mobility on pneumatic otoscopy. Left tympanic membrane appears clear with good mobility. Throat is erythematous. Thyroid, heart, lungs, genitalia, and nervous system are within normal limits; his abdomen is tender. You are not able to palpate lymph nodes, and there are no signs of hepatosplenomegaly.

What is the next step in the diagnosis of the primary disorder in this child?

A) KoH prep of skin changes
B) CBC with differential
C) Genetic testing
D) Test stool for ova and parasites
E) Tympanocentesis