Diseases and Their Causes: Organisms, Vectors and PYQs

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 C, the Aedes aegypti mosquito. This mosquito, marked by white stripes on a dark body, bites during the day and breeds in clean stored water in coolers, tyres and flower pots, which is why dengue control campaigns concentrate on emptying household containers. The same mosquito carries chikungunya, yellow fever and Zika. Option A is wrong because the female Anopheles carries malaria. Option B is wrong because the Culex mosquito carries filariasis, Japanese encephalitis and West Nile fever. Option D is wrong because the sandfly carries kala-azar, or visceral leishmaniasis, which is endemic in parts of Bihar and Jharkhand. The quickest way to keep the four straight is to memorise one vector with its set of diseases rather than one disease at a time.

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.

The correct answer is A, Vitamin A. Vitamin A is used to make rhodopsin, the pigment of the rod cells of the retina that allows sight in dim light, so a shortage of it makes a person unable to see properly at dusk or in a dark room. Prolonged deficiency dries the cornea, a condition called xerophthalmia, which can end in blindness, and carrot, papaya, green leafy vegetables, milk and liver are the usual sources. Option B is wrong because a lack of vitamin B1, thiamine, causes beriberi, with weakness of the nerves and swelling of the legs. Option C is wrong because a lack of vitamin C causes scurvy. Option D is wrong because a shortage of iron causes anaemia, in which the blood cannot carry enough oxygen and the person tires easily. The symptom described in the stem is the key to such questions.

The correct answer is B, a worm. Filariasis is caused by the thread-like nematode Wuchereria bancrofti, which is carried by the Culex mosquito. The adult worms live in the lymph vessels and block them, so fluid collects in the legs, arms or other parts and the skin thickens, producing the swelling that gives the disease its common name of elephantiasis. Option A is wrong because protozoan diseases include malaria, kala-azar, amoebic dysentery and sleeping sickness. Option C is wrong because bacterial diseases of this group include plague and tuberculosis. Option D is wrong because fungal infections of the skin are ringworm and athlete's foot, which do not block lymph vessels. Group the worm diseases together for revision: filariasis, ascariasis caused by the roundworm and taeniasis caused by the tapeworm.

The correct answer is B, the sandfly. Kala-azar, also called visceral leishmaniasis, is caused by the protozoan Leishmania donovani and is passed on by the bite of the female sandfly, which breeds in cracks of mud walls and damp soil. The disease brings long fever, weight loss, darkening of the skin and enlargement of the spleen and liver, and it occurs mainly in parts of Bihar, Jharkhand, West Bengal and Uttar Pradesh. Option A is wrong because the housefly carries typhoid, cholera and diarrhoea mechanically on its body and does not transmit Leishmania. Option C is wrong because the tsetse fly of Africa carries sleeping sickness, caused by Trypanosoma. Option D is wrong because the rat flea carries plague, caused by Yersinia pestis. Note that both kala-azar and sleeping sickness are protozoan, with different flies.

The correct answer is C, Rabies. Rabies is caused by a virus present in the saliva of an infected animal, usually a dog, and it travels along the nerves to the brain, where it causes inflammation, fear of water and almost certain death once the symptoms begin; it is prevented by vaccination after the bite, a method that goes back to Louis Pasteur. Option A is wrong because tuberculosis is caused by the bacterium Mycobacterium tuberculosis. Option B is wrong because cholera is caused by the bacterium Vibrio cholerae, which acts through a toxin in the small intestine. Option D is wrong because typhoid is caused by the bacterium Salmonella typhi and is diagnosed by the Widal test. Keep a short list of viral diseases ready: polio, measles, mumps, rabies, dengue, chickenpox, influenza, hepatitis and smallpox.

The correct answer is B, Iodine. The thyroid gland needs iodine to make the hormones thyroxine and triiodothyronine, and when the supply is short the gland enlarges in an attempt to trap more of it, which appears as a swelling in the neck. Severe deficiency in children causes cretinism, with stunted growth and reduced mental development, and the remedy adopted in India is the sale of iodised salt. Option A is wrong because a lack of calcium weakens bones and teeth and can cause spasms of the muscles. Option C is wrong because a shortage of iron causes anaemia, since iron is part of haemoglobin. Option D is wrong because a lack of vitamin D causes rickets and osteomalacia, although it indirectly affects calcium. Note that iodine is a mineral, not a vitamin, which papers sometimes test.

The correct answer is B, Haemophilia. Haemophilia is an inherited disorder in which a clotting factor is missing, so bleeding continues for a long time after an injury. The gene lies on the X chromosome, which is why the disease appears mostly in males while females usually carry it, and it is the classic example used to explain sex-linked inheritance along with colour blindness. Option A is wrong because cholera is an infectious disease caused by Vibrio cholerae spreading through water. Option C is wrong because rickets is a deficiency disease that follows a shortage of vitamin D and the poor absorption of calcium. Option D is wrong because trachoma is an infection of the eye caused by Chlamydia trachomatis and is a leading cause of preventable blindness. Other inherited disorders worth listing are sickle cell anaemia, thalassaemia and colour blindness.