“1. a Body Mass Index (BMI) of 20 to 23” → INCORRECT.

Body mass index (BMI) is supposed to relate your weight to your height. If it is over 25, you are overweight, and over 30, you are obese. The normal range is between 18.5 to 25 for men and 19 to 24 for women. Being overweight is one of the common causes of back pain, especially in the lower back. However, this statement is referring to a "normal weight".

“2. different length of the lower extremities” → CORRECT.

Those who suffer from functional leg length discrepancy are those who tend to suffer from lower back pain. The side that appears shorter often has a “lateral pelvic tilt and a shortening of the lateral flexor muscles.” This sideways pull increases compression on the facet joints for the short side and decreases the space between the vertebra which can lead to pinched nerves (sciatica).

"3. slipped intervertebral disc" → CORRECT.

A slipped disc is when a soft cushion of tissue between the bones in your spine pushes out. It's painful if it presses on nerves. A 'slipped' (prolapsed) disc often causes sudden, severe lower back pain.

"4. muscular tension and fatigue of the back muscles." → CORRECT.

This statement is describing a muscular back pain. A person can treat muscle fatigue and reduce discomfort in the lower back with rest, hot or cold therapy, over-the-counter (OTC) pain relievers, such as ibuprofen and naproxen and gentle exercises to stretch and loosen tight muscles.