Which of the following is the only artery in the human body that carries deoxygenated blood?

The correct answer is B, nephrons. The nephron is the structural and functional unit of the kidney, and each human kidney holds about one million of them. Blood is pushed into the cup shaped Bowman's capsule through a knot of capillaries called the glomerulus, and there the actual filtration happens: water, salts, glucose, amino acids and urea pass out while blood cells and large plasma proteins are held back. Option A is wrong because the ureter is only the tube that carries formed urine from the kidney down to the urinary bladder; it filters nothing. Option C is wrong because neurons are nerve cells that conduct impulses and belong to the nervous system, not the excretory system; the similar sound of nephron and neuron is exactly the trap in this question. Option D is wrong because the urethra is the last passage through which urine leaves the body. Remember the order: nephron filters, ureter carries, bladder stores, urethra releases.

The correct answer is D, group O. In the ABO system the antigens sit on the surface of the red blood cell and the antibodies float in the plasma. Group O red cells carry neither the A antigen nor the B antigen, so there is nothing on them for the recipient's antibodies to attack, and blood of this group can be given to a person of any ABO group. That is why group O is called the universal donor. Option A is wrong because group A cells carry the A antigen and would be destroyed by the anti-A antibody present in the plasma of groups B and O. Option B is wrong for the same reason with the B antigen. Option C is wrong because group AB is the opposite case: its plasma has no anti-A or anti-B antibody, so it can receive from everyone and is called the universal recipient, not the universal donor. Keep the pair straight: O gives to all, AB takes from all.

The correct answer is B, the sino-atrial node. The heartbeat is not started by a nerve from the brain; the heart generates its own rhythm, and the patch of specialised tissue that does so fastest sets the pace for the rest. That patch is the sino-atrial node, lying in the wall of the right atrium near the opening of the superior vena cava, and it is therefore called the pacemaker of the heart. Option A is wrong because the atrio-ventricular node only receives the impulse from the sino-atrial node and delays it briefly so that the atria can finish emptying; it can take over as a slower backup pacemaker but is not the normal one. Option C is wrong because the bundle of His is the conducting pathway that carries the impulse from the atrio-ventricular node into the septum between the ventricles. Option D is wrong because the Purkinje fibres are the final branches that spread the impulse through the ventricular muscle. The order to memorise is sino-atrial node, atrio-ventricular node, bundle of His, Purkinje fibres.

The correct answer is B, the spleen. A red blood cell lives about one hundred and twenty days. As it ages its membrane grows stiff, and the narrow winding channels of the spleen trap such cells so that the macrophages lining them can break the cells down and salvage the iron of their haemoglobin. Because so many worn out red cells are destroyed there, the spleen is called the graveyard of red blood cells; it is also the largest lymphoid organ of the body and acts as a blood reservoir. Option A is wrong because the liver, although it does handle the bilirubin left from broken haemoglobin and turns it into bile pigment, is not the place where the cells are trapped and destroyed. Option C is wrong because the red bone marrow is the opposite: it is the birthplace of red blood cells, not their graveyard. Option D is wrong because the pancreas makes digestive juice and the hormones insulin and glucagon and has no role with red cells at all.

The correct answer is B, the liver. When the body breaks down more protein than it needs, the amino acids are stripped of their amino group and the ammonia so released is highly poisonous. The liver converts this ammonia into urea, a far less toxic compound, through a series of reactions known as the urea cycle or the ornithine cycle. The urea then travels in the blood to the kidney, which only filters it out and concentrates it in the urine. Option A is wrong for exactly that reason: the kidney removes urea but does not make it, and this separation of the site of manufacture from the site of removal is what the question tests. Option C is wrong because the urinary bladder is only a muscular sac that stores urine until it is passed out. Option D is wrong because the spleen destroys worn out red blood cells and stores blood. Because human beings pass out mainly urea they are described as ureotelic animals, unlike ammonotelic bony fishes and uricotelic birds and reptiles.