Dengue is transmitted to human beings by

The correct answer is D, E. coli. Escherichia coli is a bacterium whose harmful strains reach the body through drinking water or food contaminated with sewage, and the infection they produce shows itself as diarrhoea and abdominal pain, which places it among the classic water-borne illnesses along with cholera, typhoid and hepatitis A. Option A is wrong because measles is a viral disease that spreads through the air, in droplets released when an infected person coughs or sneezes. Option B is wrong because dengue is spread by the bite of the Aedes aegypti mosquito, which breeds in clean stagnant water but does not pass the virus through drinking water. Option C is wrong because yellow fever is also a mosquito-borne viral disease of Africa and South America, again carried by Aedes. The lesson for the exam is to sort the options by route of spread, not by the severity of the disease.

The correct answer is A, Shigella. Bacillary dysentery, also called shigellosis, is caused by bacteria of the genus Shigella, which invade the lining of the large intestine and produce frequent stools with blood and mucus, along with fever and cramps; it spreads through contaminated water and food. Option B is wrong because Vibrio cholerae causes cholera, in which the stool is watery and rice-like rather than bloody, and the danger is rapid loss of fluid. Option C is wrong because Streptococcus pyogenes causes throat infection, scarlet fever and skin infections such as impetigo. Option D is wrong because Mycobacterium tuberculosis causes tuberculosis, which chiefly affects the lungs. Remember also that there is a second kind of dysentery, the amoebic form, caused by the protozoan Entamoeba histolytica, so read whether the question asks for a bacterium.

The correct answer is A, Only II. Rust of wheat is indeed a fungal disease, caused by species of Puccinia, and it is named from the reddish brown spores that appear in streaks on the leaves and stem. The first pair is wrong, because citrus canker is caused by a bacterium of the genus Xanthomonas, not by a virus; it produces raised corky lesions with a yellow halo on leaves and fruit. Option B is wrong because the second statement is correct, so it cannot be that neither holds. Option C is wrong because it accepts the faulty first pair and rejects the sound second one. Option D is wrong for the same reason, since it treats citrus canker as viral. A useful guide is that rusts, smuts and most rots are fungal, cankers and blights of rice and citrus are bacterial, and mosaic and leaf curl diseases are viral.

The correct answer is B, a protozoan of the genus Plasmodium. Four species of Plasmodium commonly infect human beings, of which Plasmodium falciparum is the most dangerous and Plasmodium vivax the most widespread in India. The parasite is injected when an infected female Anopheles mosquito bites, multiplies in the liver and then destroys red blood cells, which produces the cycle of chill, fever and sweating. Option A is wrong because no bacterium causes malaria, though plague and cholera are bacterial. Option C is wrong because the viral fevers spread by mosquitoes are dengue, chikungunya and Japanese encephalitis, not malaria. Option D is wrong because a roundworm, Wuchereria bancrofti, causes filariasis and is carried by the Culex mosquito. Ronald Ross, working at Secunderabad, showed the role of the mosquito in malaria.

The correct answer is B, Vitamin C. Vitamin C, or ascorbic acid, is needed to make collagen, the protein that holds tissues together, so its absence weakens the gums and blood vessels and delays the healing of wounds. It was the disease of long sea voyages until citrus fruit was added to rations, and amla, guava and lemon are rich sources. Option A is wrong because a lack of vitamin A causes night blindness and, in severe cases, xerophthalmia. Option C is wrong because a lack of vitamin D causes rickets in children and osteomalacia in adults, since the vitamin is needed to absorb calcium. Option D is wrong because a lack of vitamin K delays the clotting of blood, as it is required to make prothrombin. Vitamin C and the B group are water soluble, while A, D, E and K are fat soluble.