Body Fluids and Its Circulation — NEET UG practice

Checking statement A: erythroblastosis foetalis actually occurs when the mother is Rh-negative and the foetus is Rh-positive — statement A wrongly reverses this (says foetus is Rh-negative and mother is Rh-positive), so A is incorrect.

Checking statement B: the Rh antigen is indeed present on the RBCs of the majority (nearly 80%) of humans, so B is correct.

Checking statement C: it is true that before any blood transfusion, the Rh group must also be matched, in addition to the ABO group, to avoid destruction of red blood cells, so C is correct.

Checking statement D: Rh incompatibility genuinely occurs when a pregnant mother is Rh-negative and the foetus is Rh-positive, so D is correct.

Checking statement E: to prevent erythroblastosis foetalis in future pregnancies, anti-Rh antibodies must be administered to the mother immediately after the delivery of the first child, not the second child as stated — so E is incorrect.

Since statements A and E are the incorrect ones, hence the correct answer is C: A and E only.

Background: the sino-atrial node (SAN) is a specialised patch of tissue in the right atrium of the heart that generates electrical impulses (action potentials) on its own, without needing any external nervous stimulation.

Because it sets the rhythm and rate of the heartbeat by itself, the SAN is commonly called the pacemaker of the heart.

In a normal, healthy human at rest, the SAN generates action potentials at a rate of about 70–75 times per minute, which corresponds directly to the normal resting heart rate.

The other ranges given (120-140, 28-30, 100-110) do not match the normal resting rate of SAN-generated impulses in a healthy human.

Hence, the correct answer is C: 70-75.

Background: the heart has specific valves at each opening that prevent the backward flow of blood, and each valve is named according to the number of muscular flaps (cusps) it has.

The opening between the right atrium and the right ventricle is guarded by a valve made up of three muscular flaps, which is why it is called the tricuspid valve.

The bicuspid (mitral) valve, in contrast, guards the opening between the left atrium and left ventricle — not the right side.

The semilunar valves are located at the openings of the ventricles into the pulmonary artery and aorta, not between an atrium and ventricle, and the sino-atrial node (SAN) is not a valve at all — it is the heart's electrical pacemaker.

Hence, the correct answer is C: tricuspid valve.