Which one among the following diseases is caused due to deficiency of Vitamin D in humans?

The correct answer is D, Volt. Potential difference is the work done to move a unit charge between two points, and its SI unit is the volt. V = W/Q, so 1 volt = 1 joule per coulomb. It is measured with a voltmeter, which is always connected in parallel across the part of the circuit being tested. The unit is named after Alessandro Volta, the Italian scientist who made the first electric battery, the voltaic pile. Household supply in India is 230 V, while a pencil cell gives 1.5 V. A is wrong because the ampere is the unit of electric current, measured with an ammeter joined in series. B is wrong because the tesla is the unit of magnetic field strength (magnetic flux density). C is wrong because the coulomb is the unit of electric charge; one coulomb is the charge of about 6.25 × 10¹⁸ electrons. Exam tip: Current = ampere, Charge = coulomb, Potential difference = volt, Resistance = ohm, Power = watt, Energy = joule.

The correct answer is D, Heating effect. An iron, toaster, oven, kettle and heater all turn electrical energy into heat when current flows through a coil of high resistance. This is explained by Joule's law of heating: heat produced H = I²Rt, so the heat grows with the square of the current, with the resistance and with the time. The heating coils are made of nichrome, an alloy of nickel and chromium, because it has high resistance and does not melt or oxidise easily when red hot. The electric fuse and the filament bulb also work on this effect. A is wrong because the chemical effect of current is used in electroplating and in charging batteries. B is wrong because the magnetic effect runs electric bells, fans, motors and electromagnets. C is wrong because the physiological effect is the shock or muscle twitch that current causes in a living body. Exam tip: H = I²Rt; heater coil = nichrome, bulb filament = tungsten, fuse wire = tin-lead alloy with a low melting point.

The correct answer is C, a concave lens of suitable power. A concave lens spreads the light rays slightly before they enter the eye, so the image moves back onto the retina. In myopia, or short-sightedness, a person sees nearby objects clearly but distant objects look blurred. It happens when the eyeball is too long or the eye lens is too curved, so light from far objects meets in front of the retina. Concave lenses have negative power, so spectacles for myopia carry a minus number such as -2.0 D (dioptre). A is wrong because a biconvex lens brings rays together and would make the blur worse. B is wrong because a plain glass plate has no power and does not change where the rays meet. D is wrong because a convex lens corrects hypermetropia, or long-sightedness, in which the image forms behind the retina. Exam tip: Myopia = concave (minus) lens; Hypermetropia = convex (plus) lens; Presbyopia in old age = bifocal lens; Astigmatism = cylindrical lens.

The correct answer is B, Adrenal gland. Adrenaline is made by the adrenal glands, which sit like caps on top of the two kidneys. It is released in fear, anger or sudden danger, so it is called the "fight or flight" hormone. It raises the heartbeat and breathing rate, increases blood sugar and sends more blood to the muscles so that the body can react fast. Adrenaline comes from the inner part of the gland, the adrenal medulla; the outer part, the cortex, makes cortisol and aldosterone. A is wrong because the thyroid gland in the neck makes thyroxine, which controls metabolism and needs iodine. C is wrong because the pineal gland in the brain makes melatonin, which controls the sleep-wake cycle. D is wrong because the pituitary gland makes growth hormone and controls several other glands, so it is called the master gland. Exam tip: Adrenal = adrenaline (emergency hormone), Thyroid = thyroxine (metabolism), Pancreas = insulin (blood sugar), Pituitary = master gland.

The correct answer is D, Nucleus. The nucleus is called the master or control centre of the cell because it directs all the activities of the cell. It holds the chromosomes, which carry DNA, the genetic instructions for making proteins and for passing traits from parents to children. It is wrapped in a double membrane called the nuclear envelope, whose pores let messages move in and out. Robert Brown discovered the nucleus in 1831. Mature red blood cells of mammals have no nucleus. A is wrong because mitochondria release energy as ATP and are called the powerhouse of the cell. B is wrong because the endoplasmic reticulum is a network of membranes that makes and carries proteins and fats. C is wrong because the nucleolus is a small body inside the nucleus that makes ribosomes, so it is only a part of the control centre. Exam tip: Nucleus = control centre, Mitochondria = powerhouse, Ribosome = protein factory, Lysosome = suicide bag, Golgi body = packaging unit.