A 6-month-old girl is brought to the pediatrician by her father for a scheduled check-up. She was in her usual state of health until 6 weeks ago when she was hospitalized for bacterial pneumonia. She received amoxicillin and had a resolution of her symptoms. Her father reports that she seems “back to normal.” She can roll over by herself and can sit up without support. She often babbles but sometimes does make identifiable sounds like “dada” and “baba.” They recently started feeding her mushy cereal, which she seems to like but occasionally spits up. She has started teething and sometimes becomes “fussy.” Vital signs are within normal limits. A physical examination is unremarkable. The patient’s father brought a 4-week follow-up chest radiograph taken at the hospital, as shown in Figure A. Which of the structures labeled in Figure A would be most likely to change in response to another infectious illness?
A
B
C
D
A
The thymus gland (labeled in A) is visible on a chest radiograph in infants and will change in response to illness due to its immunologic role.

The thymus gland is the site of T-cell maturation. The thymus is visible on a chest radiograph in infants and continues to grow throughout childhood. It is located in the anterior part of the superior mediastinum. On a chest radiograph, the thymus may appear as a “widened mediastinum,” continuous with the superior border of the heart (Figure A) or as a triangular-shaped opacity known as the thymic “sail sign," located towards the right of the mediastinum. At puberty, the gland will decrease in size and eventually be replaced by fatty tissue.

Askin and Young review the evidence regarding the history and function of the thymus. They discuss how this structure is involved in immune function. They recommend monitoring for the absence of this structure in immunodeficient patients.

Figure A shows a normal chest radiograph of an infant with a normal cardiothymic contour.

Incorrect Answers:
Answer B: B points to the trachea. Deviation of this structure can be seen in patients with tension pneumothorax. Treatment is with needle thoracostomy followed by chest tube placement.

Answer C: C points to the apex of the heart. This structure can be enlarged in congenital heart conditions. Treatment is with surgical correction of these conditions.

Answer D: D points to the stomach. This structure can be abnormal in patients with pyloric stenosis. Treatment is with pyloromyotomy.

Answer E: E points to the diaphragm. This structure can be abnormal in patients with diaphragmatic hernias. Treatment is with surgical closure of the defect.

Bullet Summary:
The thymus is normally visible on a pediatric chest radiograph, but it is replaced by fatty tissue after puberty.