How many pairs of cranial nerves are present in the human body?

The correct answer is B, the brain. The brain is the organ that interprets pain, but its own tissue has no pain receptors, so cutting or pricking it causes no sensation; the covering membranes, the blood vessels and the scalp do have receptors, which is why a headache hurts. This is the reason brain surgery can be performed on a patient who is awake and talking, with only the scalp anaesthetised. Option A is wrong because the skin is the richest field of pain receptors in the body and a needle prick there is the standard example of a painful stimulus. Option C is wrong because the heart has pain receptors and signals its distress as the crushing pain of angina, often felt in the left arm as referred pain. Option D is wrong because the cornea of the eye is among the most sensitive tissues of the body, which is why the smallest speck of dust in it is unbearable.

The correct answer is B, the thyroid gland, a butterfly-shaped gland in front of the windpipe in the neck, which is the largest purely endocrine gland in the body and secretes thyroxine, triiodothyronine and calcitonin. Option A is wrong because the pituitary is the smallest endocrine gland, about the size of a pea, although it is the master gland that controls several others, so the two make a standard pair of opposites in exams. Option C is wrong because the adrenal glands, one above each kidney, are small, and they are remembered for adrenaline rather than for size. Option D is wrong on a technicality worth knowing: the pancreas is larger than the thyroid, but it is a mixed or heterocrine gland, part exocrine for digestive juice and part endocrine for insulin, so the largest purely endocrine gland remains the thyroid.

The correct answer is B, the cerebellum, which lies below and behind the cerebrum and coordinates the work of the muscles so that movement is smooth and the body stays balanced; this is why a person whose cerebellum is affected by alcohol staggers. Option A is wrong because the cerebrum, the largest part of the brain, is the seat of thought, memory, intelligence and the will to move, but not of the fine coordination that follows the order. Option C is wrong because the medulla oblongata governs the involuntary actions that keep a body alive, the heartbeat, breathing, blood pressure, vomiting, sneezing and coughing. Option D is wrong because the thalamus is a relay station that sorts sensory impulses and passes them on to the cerebrum. Keep the trio apart: cerebrum for thinking, cerebellum for balance and medulla for the involuntary.

The correct answer is B, the pituitary, a pea-sized gland lodged in a bony socket at the base of the skull and attached to the hypothalamus. It is called the master gland because its secretions control other glands: thyroid stimulating hormone drives the thyroid, adrenocorticotropic hormone the adrenal cortex, and the gonadotropins the testes and ovaries, besides its own growth hormone. Option A is wrong because the thyroid is itself under the pituitary's command, and its claim to fame is being the largest endocrine gland. Option C is wrong because the pineal gland secretes melatonin and keeps the daily rhythm of sleep, a narrower task. Option D is wrong because the thymus secretes thymosin for the training of the immune cells and shrinks after puberty, so it neither controls other glands nor lasts a lifetime.

The correct answer is B, the beta cells of the islets of Langerhans in the pancreas. Insulin lowers the level of glucose in the blood by helping the cells take it in and by having the liver store it as glycogen, and a want of insulin or a failure of the body to respond to it causes diabetes mellitus. Option A is wrong because the alpha cells of the same islets make glucagon, which does the opposite and raises blood sugar; the two hormones together keep the level steady, and swapping them is the commonest error in this question. Option C is wrong because the adrenal medulla secretes adrenaline and noradrenaline; adrenaline also raises blood sugar, but it is not insulin. Option D is wrong because the thyroid follicles make thyroxine, which sets the rate of metabolism and contains iodine.