What micronutrient deficiency is associated with alopecia, anosmia, brittle nails, and chronic diarrhea?

The micronutrient deficiency associated with alopecia, anosmia, brittle nails, and chronic diarrhea is zinc. Zinc plays a critical role in various bodily functions, including immune system support, cellular metabolism, and protein synthesis. Its deficiency can lead to a range of symptoms, including hair loss (alopecia), loss of smell (anosmia), brittle nails, and gastrointestinal disturbances such as chronic diarrhea.

Alopecia can occur due to the impaired function of hair follicle cells, which rely on adequate zinc levels for growth and maintenance. Similarly, anosmia may arise because zinc is involved in the proper functioning of sensory receptors in the nasal cavity. Brittle nails can result from their structural weakness without sufficient zinc, and chronic diarrhea is often linked to zinc's role in maintaining the health of the intestinal lining and immune function.

Each of the other listed micronutrients has distinct roles and associated deficiencies, which do not encompass the full range of symptoms mentioned in the question. Thus, a deficiency in zinc specifically aligns with the symptoms presented, making it the correct answer.