A morbidly obese toddler who weighs nearly four stone may be suffering from an incredibly rare genetic disease, doctors believe.

Aliya Saleem, who is just 18 months old, drew worldwide attention earlier this year when a video of her incredible size received more than 21 million views on YouTube.

Since then, she has continued to gain weight and is now almost as heavy as an average eight-year-old - weighing 3st 11lbs (53lbs or 24kg)

This makes her one of the heaviest girls in the world for her age.

Initially, her mother Shabnam Parveen, 25, and father Mohammad Saleem, 28, were baffled by what was causing their daughter's insatiable appetite.

But now, a doctor believes Aliya may be suffering from Prader-Willi syndrome, a rare disorder characterised by learning difficulties, growth abnormalities, and obsessive eating.

Dr Bhavya Kumar, paediatrician at the Orchid Medical Centre, in Ranchi, says this is the most likely cause of her weight gain, among many theories.

Aliya had a sister, Simran, who died of a brain haemorrhage, and also excessively ate, suggesting a genetic link.

He said: 'There could be some hormonal imbalance, there could some metabolic disease. There can be some genetic causes.

'It could be because of some tumour. But because there is a history of sibling loss with similar complaints, that points towards a genetic cause.

'So, I strongly suspect that this could be a case of some genetic disorder, most probably Prader-Willi syndrome, which is a very rare disorder.'

However, in order to make a definitive diagnosis, Dr Kumar has a number of other tests he would have to run on Aliya.

He said: 'We would like to get her hormone levels checked. We would like to get her organ function test done so that we can see that all her organs are functioning properly.

'We would also like to screen her for diabetes, a thyroid function test, her cortisol levels need to be done - and we need to see her growth hormone levels as well.

'We will do an MRI scan to see her hypothalamus, an ultrasound to see her adrenals. And apart from that, an X-ray to see her bones.'

He added Aliya would also benefit from karyotyping - a lab technique to look for any major chromosomal anomaly which may cause a genetic condition.

'That would rule out any genetic causes,' he explained. 

'And only after doing all those things, we would be probably able to make the diagnosis and then we can take it further from there.'

Aliya lives with her family in a remote Indian village in the state of Jharkhand, and travels to the capital Ranchi for medical treatment.

She weighed roughly 9lbs (4kg) at birth, but started ballooning in size three months later.

At nearly four stone (25kg), she now weighs roughly double the average weight of a two-year-old at just 18 months.  

Mrs Parveen and Mr Saleem find her condition especially painful after another one of their daughters died in similar circumstances.

She said: 'Simran too suffered from weight issues and ate excessively, just like Aliya.

The family had shifted to Gujarat for Simran's treatment, but the doctors could not pinpoint the cause for the excessive weight gain.

Aliya requires three times the amount of food of a normal child her age, Mrs Parveen said.

Her parents have previously told how they have to feed their daughter constantly - as otherwise she breaks down in tears and throws temper tantrums.

Mr Saleem works as a tailor and brings in roughly £2 a day - with a huge portion of his wages going towards feeding his daughter.

The couple also have a five-year-old son called Ali who is a regular weight.

Mrs Parveen said: 'We spend all our money on feeding Aliya.

'If we do not give her something to eat, she cries endlessly and gets cranky.

'Her constant hunger leads to excessive weight gain due to which we have to buy new sets of clothes every two weeks that take a toll on our limited expenses.'