Answer: Option A. -> Nephridia
:
A
Nephridia are the tubular excretory structures of earthworms and other annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance. Malpighian tubules are the excretory structures of most of the insects including cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation. Antennal glands or green glands perform the excretory function in crustaceans like prawns. Vertebrates have complex tubular organs called kidneys.

Answer: Option D. -> I only
:
D
The only true statement here is that urine is isotonic in the proximal convoluted tubule (PCT).
One of the important aspects of elimination of wastes from the body is the maintenance of body fluid and electrolyte balance. The body cannot afford to lose all the salts and electrolytes.Therefore the kidneys have a special mechanism to concentrate urine, it is called countercurrent mechanism, which occurs in the loop of Henle. The animals which excrete hypertonic urine havelonger loops of Henle. The loop of Henle dips into the medullary region of the kidneys, which has aconcentration gradient ranging from about 300mOsmols to 1200mOsmols from the cortex to the inner medulla. This osmolarity gradient in theinterstitial fluidis maintained by the countercurrent mechanism. The urine before it reaches the loop of Henle, that is in the PCT is isotonic. However, as it travels down the descending loop of Henle the urinegets increasingly concentrated, because it loses water to the highly concentrated interstitium,through the water permeable channels of the descending duct. Thus urine at the U-turn of the loop is highly concentrated, at about 1200mOsmols. Now the ascending loop is completely impermeable to water and highly permeable to sodium. Therefore as the urine travels up the ascending limb, the sodium and chloride ions diffuse out of the ascending limb through the sodium and calcium-permeable channels. Thus urine loses most of its salt as it travels up the ascending loop becoming more hypotonic until it is isotonic once again at the DCT.
Therefore urine becomes more and more hypertonic as it passes down the Henle's loop, but becomesisotonic once again as it gets tothe DCT, and when it enters the collecting tubule.

Answer: Option C. -> A salt such as an oxalate crystallised in minor/major calyces, pelvis or the ureter.
:
C
Akidney stoneis a solid, pebble-like piece of material that can form in one or both of your kidneys when high levels of certain minerals are in your urine.So a kidney stone can be crystallised chemicals like uric acid, calcium oxalate and calcium phosphate.

Answer: Option A. -> Proximal convoluted tubule and Mitochondria
:
A
The first part of the tubule, just after the Bowman's capsule, is the proximal convoluted tubule. Brush border cells with multiple microvilli and a massive amount of mitochondria line this tube. This portion of the tubule is one of the largest contributors to electrolyte and water reabsorption. Around 65% of the sodium and water and a little less chloride that were filtered out of the plasmais reabsorbed into the vasa recta in this section.

Answer: Option D. -> Renal tubules
:
D
The Bowman's capsule has an outer epithelium of simple squamous epithelial cells called the parietal epithelial layer and an inner epithelial layer of what are called visceral cells. These cells fuse with the endothelial cells lining the glomerular capillaries. In between the endothelial cells and the visceral cells lies an additional layer called the glomerular basement membrane. All three of these layers together, form the glomerular filtration barrier.

Answer: Option C. -> Resins and Gums
:
C
Coniferous trees excrete waste products such as resins and gums which have high economical value. Resins are valued for their chemical properties and associated usessuch as the production ofvarnishes,adhesivesand foodglazing agents. They are also prized as an important source of raw materials fororganic synthesisand provide constituents ofincense and perfumes.

Answer: Option D. -> Aldosterone and ADH
:
D
In mammals and birds, the amount of water excreted in the urine, and thus the concentration of the urine, varies according to the changing needs of the body. Volume of urine is regulated by ADH and aldosterone.The osmotic gradient is normally constant, but the permeability of the collecting duct to water is adjusted by a hormone, antidiuretic hormone, ADH. Aldosterone maintains salt balance in the body.

Answer: Option D. -> The urine will be more dilute
:
D
Henle's loop is associated with reabsorption of water and electrolytes making the urine more concentrated.Special arrangement of Henle’s loop and vasa recta is called the counter current mechanism. This mechanism helps to maintain a concentration gradientin the medullary interstitium. Presence of such interstitial gradient helps
in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.

Answer: Option C. -> Afferent glomerular arteriole is wider than efferent glomerular arteriole
:
C
The diameter of the afferent (incoming) arteriole is greater than the diameter of the efferent arteriole (by which blood leaves the glomerulus). As a result, there is no drop in blood pressure that would usually occur when blood flows from the arterial end to the venous end in capillaries. The maintenance of high pressure drives glomerular filtration.

Answer: Option D. -> Hydrochloric acid
:
D
Mammalssuch ashumansexcrete urea and ammonia while birds,reptilesand some terrestrial invertebrates produce uric acidas waste. Uricotelic organisms tend to excreteuric acid waste in the form of a white paste or powder. Hydrochloric acid isnotthe wasteproductexcretedby the animal.