The following are antigen presenting cells EXCEPT

Correct Answer: D

Rationale: Antigen-presenting cells (APCs) process and display antigens to T-cells, initiating adaptive immunity. 'Natural killer cells,' is correct as the exception, per immunology references like Abbas' Basic Immunology, as NK cells target infected cells directly, not presenting antigens. 'Macrophages,' engulf pathogens and present antigens via MHC. 'Dendritic cells,' are potent APCs, activating naive T-cells. 'B-Lymphocytes,' present antigens to helper T-cells for antibody production. NK cells' innate cytotoxic role, lacking MHC presentation, distinguishes D as the accurate answer, separate from APC functions.

Correct Answer: C

Rationale: Essential amino acids must be dietary, as the body cannot synthesize them. 'Alanine,' is correct as the exception, a non-essential amino acid produced from pyruvate, per biochemistry (e.g., Nelson). 'Leucine,' 'Lysine,' and 'Valine,' are essential, required from food. Alanine's endogenous synthesis contrasts with the dietary need for others, making C the precise answer.

Correct Answer: C

Rationale: Villi are the small intestine's functional units, maximizing absorption. 'Villi,' is correct, per histology (e.g., Ross), with enterocytes for digestion and uptake. 'Mucosa,' is a layer containing villi. 'Muscularis,' aids motility. 'Serosa,' is the outer layer. Villi's absorptive role makes C the precise answer.

Correct Answer: B

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.

Correct Answer: B

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.