Knock knee syndrome is caused by
(A) Fluoride pollution
(B) Lead pollution
(C) Mercury pollution
(D) Arsenic pollution

Hint: Genu valgum, also known as "knock-knee," is a condition in which the knees angle inward and touch when the legs are straightened. People with severe valgus deformities are usually unable to touch their feet together while straightening their legs.

Complete answer:
Fluoride pollution causes knock-knee syndrome. Knock knee syndrome, also known as rickets or Genu Valgum, can lead to vitamin D deficiency. Knock knee can be caused by a congenital or developmental disease, or it can develop as a result of an infection or a traumatic knee injury.

The following are some of the most common causes of knock knees:
- metabolic disorder
- kidney (renal) failure
- physical harm (injury)
- Infection of the knee bones, in particular, causes arthritis (osteomyelitis)
- Rickets is a type of ricket (a bone disease caused by lack of vitamin D)
- congenital (inborn) disorders
- injury to the growth plate
- Bone tumors are benign fractures that heal with a deformity (malunion)

When a person's knees are positioned together, the most noticeable symptom of knock knee is a separation of their ankles. Other symptoms, such as pain, are frequently caused by the gait (style of walking) adopted by people with knock knees.

Other symptoms may occur as a result of the underlying condition that is causing the knock-knee syndrome. One or both knees are abnormally overloaded in people of all ages who have knock knees.

Thus, the answer is Option ‘A’ i.e, Fluoride pollution.

Note: Bracing may help to reposition the knees in children and adolescents with mild knock knees. When this fails, or if the patient is an adult at the time of diagnosis, a knee-realignment osteotomy is performed to avoid or postpone the need for knee replacement.