Final answer:
The simplest lymphoid organs at the entrance to the pharynx are the tonsils. They form part of the secondary lymphoid organs and are crucial in the body's defense against pathogens entering through the mouth and nose.
Step-by-step explanation:
The simplest lymphoid organs found at the entrance to the pharynx are the tonsils. The tonsils are part of the secondary lymphoid tissue which also includes spleen, lymph nodes, and some structures like Peyer's patches and mucosa-associated lymphoid tissue (MALT).
Tonsils are present in four pairs, and they encircle the part of the anatomy where the respiratory and gastrointestinal tracts intersect. This makes them well-placed for detecting and fighting pathogens that enter through the mouth and nose. This collection of tonsil tissue is referred to as Waldeyer's ring. The spleen is a secondary lymphoid organ with a role in filtering blood and lymph, among other functions.