Write SQL query to solve given problem: List out 5 most common conditions for underweight patient.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT DISTINCT T2.DESCRIPTION, T2.VALUE, T2.UNITS FROM patients AS T1 INNER JOIN observations AS T2 ON T1.patient = T2.PATIENT WHERE T2.DESCRIPTION = 'Body Mass Index' GROUP BY T2.VALUE ORDER BY COUNT(T2.VALUE) LIMIT 5