Assertion (A): Red blood cells lack a nucleus in mammals. Reason (R): The absence of a nucleus allows more space for hemoglobin, increasing oxygen-carrying capacity. MCQ with Answer and Explanation

Explanation:
Kala-azar, also known as visceral leishmaniasis, is a severe systemic disease caused by the protozoan parasite Leishmania donovani. The disease is transmitted to humans through the bite of an infected female Phlebotomus sandfly (in the Old World) or Lutzomyia sandfly (in the New World). The sandfly injects the infective promastigote stage of the parasite into the skin during a blood meal. Anopheles transmits malaria, Aedes transmits dengue and yellow fever, and Culex transmits filariasis.

Explanation:
Prokaryotic DNA is typically circular, double-stranded, and found in a nucleoid region without a nuclear membrane. It is not associated with histones (though some Archaea have histone-like proteins). Plasmids are extra-chromosomal DNA. Eukaryotic DNA is linear and complexed with histones. This difference is fundamental in cell classification.

Explanation:
Iodine atoms are incorporated into thyroxine (T₄, 4 iodine atoms) and triiodothyronine (T₃, 3 iodine atoms) synthesized by thyroid follicular cells. Iodine deficiency leads to goiter and hypothyroidism. Iron is in hemoglobin, calcium in bones, phosphorus in ATP and bones.