What is the most likely diagnosis for a 12-month-old girl presenting with cough, mild subcostal retractions, and asymmetric wheezing?

The presentation of a 12-month-old girl with cough, mild subcostal retractions, and asymmetric wheezing strongly suggests a diagnosis of foreign body aspiration. In young children, foreign bodies can easily become lodged in the airway, causing localized symptoms such as asymmetric wheezing due to the obstruction of airflow in one lung more than the other.

Typically, when a foreign body is present, the child may display sudden onset of coughing, respiratory distress, or wheezing that is often not symmetrical, reflecting the unilateral nature of the obstruction. The presence of mild subcostal retractions indicates that the child is working harder to breathe, which is common in cases of airway obstruction.

In contrast, other potential diagnoses do not align as neatly with the asymmetric wheezing seen in this case. Asthma exacerbation can cause wheezing but generally presents in a more symmetrical manner and in older children. Pneumonia typically leads to more diffuse wheezing along with fever and other systemic signs, and bronchitis is usually characterized by a more generalized pattern of wheezing rather than localized symptoms.

The combination of the child's age, symptomatology, and the acute nature of her respiratory distress points towards foreign body aspiration as the most likely diagnosis.