**Core Concept**
DiGeorge syndrome, also known as 22q11.2 deletion syndrome, is a congenital disorder caused by a defect in chromosome 22. It primarily affects the development of the third and fourth pharyngeal pouches, leading to various clinical manifestations.

**Why the Correct Answer is Right**
DiGeorge syndrome is characterized by the failure of development of the third and fourth pharyngeal pouches, which are responsible for the formation of several key structures, including the thymus, parathyroid glands, and parts of the aortic arch. However, the primary immune defect in DiGeorge syndrome is not related to B cells but rather to T cells, as the thymus is underdeveloped or absent. This results in T cell immunodeficiency, making patients susceptible to infections and autoimmune disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** The failure of development of the third and fourth pharyngeal pouches is indeed a characteristic of DiGeorge syndrome, making this option incorrect.
**Option B:** Absence of the thyroid gland is not typically associated with DiGeorge syndrome. The thyroid gland develops from the thyroid diverticulum, which is separate from the third and fourth pharyngeal pouches.
**Option C:** Absence of parathyroid glands is a common feature of DiGeorge syndrome, as they develop from the third pharyngeal pouch. This leads to hypoparathyroidism and associated hypocalcemia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that DiGeorge syndrome can present with a wide range of clinical features, including congenital heart defects, cleft palate, and characteristic facial features. The presence of hypocalcemia and T cell immunodeficiency should raise suspicion for this condition.

**✓ Correct Answer: D. B cell defect. B cell defect is not a characteristic feature of DiGeorge syndrome.**