**Core Concept**
Histiocytosis refers to a group of disorders characterized by the abnormal accumulation of histiocytes, a type of immune cell, in various organs. This accumulation can lead to tissue damage and organ dysfunction. The most common site of histiocytosis varies depending on the type of disorder.

**Why the Correct Answer is Right**
Langerhans cell histiocytosis (LCH) is the most common form of histiocytosis, and it typically affects the bones, particularly the skull, pelvis, and long bones. The involvement of the bones is due to the accumulation of Langerhans cells, a type of dendritic cell, in the bone marrow and surrounding tissues. This leads to bone destruction and can cause symptoms such as bone pain, fractures, and lytic lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** The pituitary gland is a common site of involvement in certain types of histiocytosis, such as Hand-Schüller-Christian disease, but it is not the most common site overall.

**Option B:** The liver is not typically a primary site of histiocytosis, although it can be involved in some cases, particularly in the context of systemic disease.

**Option C:** The lungs can be involved in certain types of histiocytosis, such as pulmonary Langerhans cell histiocytosis, but this is not the most common site overall.

**Clinical Pearl / High-Yield Fact**
Langerhans cell histiocytosis often presents with a triad of symptoms: bone lesions, exophthalmos, and diabetes insipidus. This classic triad is a key diagnostic clue for clinicians.

**Correct Answer:** B.