Which of the following would reduce the glomerular filtration rate?
a. vasoconstriction of the efferent arteriole
b. a drop in oncotic pressure
c. vasodilation of the afferent arteriole
d. vasoconstriction of the afferent arteriole
e. an increase in osmotic pressure in the glomerular capsule

d. Vasoconstriction of the afferent arteriole.

Explanation:

The glomerulus present in the kidney filters blood at a certain rate. This rate is called the glomerular filtration rate.

Normally there is no change in glomerular filtration rate if minor fluctuation in the daily blood pressure. Our body precisely controls this minor change.

However, if there is a major drop in blood pressure due to loss of blood. Under such circumstances, the nervous system stimulates vasoconstriction (contraction) of the afferent arteriole which allows less blood flow to the glomerulus.

This will reduce the glomerular filtration rate and finally and at the end, less urine will produce.