**Core Concept**
The question is testing the common causes of respiratory infections with wheezing in children, particularly in the context of viral pathogens. Respiratory syncytial virus (RSV) and other viral agents are known to cause wheezing in young children.

**Why the Correct Answer is Right**
RSV is the most common cause of wheezing in children under the age of 2 years. It primarily infects the lower respiratory tract, causing inflammation and wheezing. RSV binds to the sialic acid receptors on the surface of epithelial cells in the respiratory tract, leading to cell fusion and the formation of syncytia, which disrupts the normal architecture of the airways and leads to wheezing.

**Why Each Wrong Option is Incorrect**
**Option A:** Adenovirus can cause respiratory infections, but it is not the most common cause of wheezing in children under 2 years. Adenovirus typically causes a more upper respiratory tract infection.
**Option B:** Influenza virus can also cause respiratory infections, but it is not the most common cause of wheezing in young children. Influenza virus tends to cause more severe illness but with less wheezing.
**Option C:** Parainfluenza virus can cause croup, a condition characterized by a barking cough, but it is not the most common cause of wheezing in children under 2 years.

**Clinical Pearl / High-Yield Fact**
RSV infection in young children often leads to recurrent wheezing, which may be a precursor to the development of asthma later in life.

**Correct Answer: D. RSV (Respiratory Syncytial Virus)**